drm/radeon: add W|RREG32_IDX for MM_INDEX|DATA based mmio accesss

Just refactoring to make the next patche simpler. Now all indirect register
access in the new modesetting driver should go through the r100_mm_(w|r)reg
fucntions.

RADEON_READ_MM from the old driver seems to be totally unused, so just kill
it.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>