drm/amdgpu: Modify comments of vram_start/end

Modify the comment to reflect the fact that, if
use GART for vram address translation for vmid0,
[vram_start, vram_end] will be placed inside SYSVM
aperture, together with GART.
Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Christian Konig <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.h

@@ -190,10 +190,13 @@ struct amdgpu_gmc {
 	u64			gart_end;
 	/* Frame buffer aperture of this GPU device. Different from
 	 * fb_start (see below), this only covers the local GPU device.
-	 * Driver get fb_start from MC_VM_FB_LOCATION_BASE (set by vbios)
-	 * and calculate vram_start of this local device by adding an
-	 * offset inside the XGMI hive.
-	 * Under VMID0, logical address == MC address
+	 * If use FB aperture to access FB, driver get fb_start from
+	 * MC_VM_FB_LOCATION_BASE (set by vbios) and calculate vram_start
+	 * of this local device by adding an offset inside the XGMI hive.
+	 * If use GART table for VMID0 FB access, driver finds a hole in
+	 * VMID0's virtual address space to place the SYSVM aperture inside
+	 * which the first part is vram and the second part is gart (covering
+	 * sram)
 	 */
 	u64			vram_start;
 	u64			vram_end;