drm/amdgpu/gmc9: don't keep stolen memory on vega12

vega12 does not appear to be affected by the same issue
as vega10.  Enable the full stolen memory handling on
vega12.  Reserve the appropriate size at init time to avoid
display artifacts and then free it at the end of init once
the new FB is up and running.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c

@@ -707,8 +707,9 @@ static bool gmc_v9_0_keep_stolen_memory(struct amdgpu_device *adev)
 	switch (adev->asic_type) {
 	case CHIP_RAVEN:
 		return false;
-	case CHIP_VEGA10:
 	case CHIP_VEGA12:
+		return false;
+	case CHIP_VEGA10:
 	case CHIP_VEGA20:
 	default:
 		return true;