drm/doc: Add VKMS module description and use to "Testing and Validation"

Add a description on VKMS module and the cases in which it should be used.
There's a brief explanation on how to set it and use it in a VM, along with
an example of running an igt-test.

Changes since V3:
 Rodrigo:
 - Change the log message to imperative
 - Fix some bad spelling/writing
 - Add a blank line before enumeration

Changes since V2:
 Andre:
 - Avoid repetition of words in the same sentence;
 - Make the explanation on 'setting the kernel' shorter, eliminate the
   'make menuconfig' command;
 - Add tab on enumeration to have one line per item;
 - Clarify from each machine igt-tests commands should be ran on.
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Gabriela Bittencourt <gabrielabittencourt00@gmail.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191104162705.19735-1-gabrielabittencourt00@gmail.com

Documentation/gpu/drm-uapi.rst

@@ -257,6 +257,43 @@ core don't regress existing functionality. This test suite is called IGT and
 its code and instructions to build and run can be found in
 https://gitlab.freedesktop.org/drm/igt-gpu-tools/.
 
+Using VKMS to test DRM API
+--------------------------
+
+VKMS is a software-only model of a KMS driver that is useful for testing
+and for running compositors. VKMS aims to enable a virtual display without
+the need for a hardware display capability. These characteristics made VKMS
+a perfect tool for validating the DRM core behavior and also support the
+compositor developer. VKMS makes it possible to test DRM functions in a
+virtual machine without display, simplifying the validation of some of the
+core changes.
+
+To Validate changes in DRM API with VKMS, start setting the kernel: make
+sure to enable VKMS module; compile the kernel with the VKMS enabled and
+install it in the target machine. VKMS can be run in a Virtual Machine
+(QEMU, virtme or similar). It's recommended the use of KVM with the minimum
+of 1GB of RAM and four cores.
+
+It's possible to run the IGT-tests in a VM in two ways:
+
+	1. Use IGT inside a VM
+	2. Use IGT from the host machine and write the results in a shared directory.
+
+As follow, there is an example of using a VM with a shared directory with
+the host machine to run igt-tests. As an example it's used virtme::
+
+	$ virtme-run --rwdir /path/for/shared_dir --kdir=path/for/kernel/directory --mods=auto
+
+Run the igt-tests in the guest machine, as example it's ran the 'kms_flip'
+tests::
+
+	$ /path/for/igt-gpu-tools/scripts/run-tests.sh -p -s -t "kms_flip.*" -v
+
+In this example, instead of build the igt_runner, Piglit is used
+(-p option); it's created html summary of the tests results and it's saved
+in the folder "igt-gpu-tools/results"; it's executed only the igt-tests
+matching the -t option.
+
 Display CRC Support
 -------------------