drm/radeon: disable the GRPH block when we disable the crtc

Since we aren't using it when the crtc is disabled, turn it off
to save power.  The GRPH block is the part of the display
controller that controls the primary graphics plane (size,
address, etc.).

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/atombios_crtc.c
Simple merge