Revert "drm/radeon: adding synchronization for GRBM GFX"

This reverts commit 1c0a4625. Not needed
anymore, since amdkfd is no longer supported with radeon.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>