drm/amdgpu: use $(src) in Makefile (v2)

BugLink: http://bugs.launchpad.net/bugs/1546572

This can solve the path problem when compile amdgpu with DKMS.

v2: agd: rebase on current drm-next
Signed-off-by: Jammy Zhou <Jammy.Zhou@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 288912cb)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/gpu/drm/amd/amdgpu/Makefile

@@ -2,10 +2,12 @@
 # Makefile for the drm device driver.  This driver provides support for the
 # Direct Rendering Infrastructure (DRI) in XFree86 4.1.0 and higher.
 
-ccflags-y := -Iinclude/drm -Idrivers/gpu/drm/amd/include/asic_reg \
-	-Idrivers/gpu/drm/amd/include \
-	-Idrivers/gpu/drm/amd/amdgpu \
-	-Idrivers/gpu/drm/amd/scheduler
+FULL_AMD_PATH=$(src)/..
+
+ccflags-y := -Iinclude/drm -I$(FULL_AMD_PATH)/include/asic_reg \
+	-I$(FULL_AMD_PATH)/include \
+	-I$(FULL_AMD_PATH)/amdgpu \
+	-I$(FULL_AMD_PATH)/scheduler
 
 amdgpu-y := amdgpu_drv.o