drm/radeon/kms: compute GPU addresses correctly on evergreen

There are also two fixes:
- In DRAW_INDEX_2, we read idx_value, but should have read idx+1.
- When correcting SQ_VTX_CONSTANT_WORD1_0.SIZE, we should subtract
  the offset.

Signed-off-by: Marek Olšák <maraeo@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>