drm/radeon: Remove radeon_gart_restore()

Doesn't seem necessary, the GART table memory should be persistent.

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