drm/amdgpu: Add RLC_PG_DELAY_3 for Vangogh

Driver should enable the CGPG feature for RLC in safe mode to
prevent any misalignment or conflict in middle of any power
feature entry/exit sequence.
Achieved by setting RLC_PG_CNTL.GFX_POWER_GATING_ENABLE = 0x1,
and RLC_PG_DELAY_3.CGCG_ACTIVE_BEFORE_CGPG to the desired CGPG
hysteresis value in refclk count.

Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Jinzhou Su <Jinzhou.Su@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>