drm/amdgpu/gmc8: use the vram location programmed by the vbios

This makes mc programming much simpler in future patches.

Since evergreen, the vbios has been programming the fb location
to the proper vram size.  The only reason to reprogram it would
be to change the location.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c

@@ -404,13 +404,16 @@ static int gmc_v8_0_polaris_mc_load_microcode(struct amdgpu_device *adev)
 static void gmc_v8_0_vram_gtt_location(struct amdgpu_device *adev,
 				       struct amdgpu_mc *mc)
 {
+	u64 base = RREG32(mmMC_VM_FB_LOCATION) & 0xFFFF;
+	base <<= 24;
+
 	if (mc->mc_vram_size > 0xFFC0000000ULL) {
 		/* leave room for at least 1024M GTT */
 		dev_warn(adev->dev, "limiting VRAM\n");
 		mc->real_vram_size = 0xFFC0000000ULL;
 		mc->mc_vram_size = 0xFFC0000000ULL;
 	}
-	amdgpu_vram_location(adev, &adev->mc, 0);
+	amdgpu_vram_location(adev, &adev->mc, base);
 	adev->mc.gtt_base_align = 0;
 	amdgpu_gtt_location(adev, mc);
 }