drm/amdgpu: replace indirect mmio access in non-dc code path

all the mmCUR_CONTROL instances are in mmr range and
can be accessd directly by using RREG32/WREG32

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