Switch over to using internal atomic API for mode set.

This removes the legacy set_config API, replacing it with
drm_atomic_helper_set_config().  The DRM helper will use various
vmwgfx-specific atomic functions to set a mode.

DRIVER_ATOMIC capability flag is not yet set, so the user mode
will still use the legacy mode set IOCTL.

v2:
* Avoid a clash between page-flip pinning and setcrtc pinning, modify
the page-flip code to use the page-flip helper and the atomic callbacks.
To enable this, we will need to add a wrapper around atomic_commit.

* Add vmw_kms_set_config() to work around vmwgfx xorg driver bug
Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>