drm/radeon: Allow setting shader registers using DMA/COPY packet3 on SI.

Mesa uses a COPY_DATA packet to copy the grid size for indirect dispatches
into COMPUTE_USER_DATA_*.

Setting those registers with a SET_SH_REG packet is allowed, not allowing
them with other packets seems like an oversight.

v2: Clarify commit message.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>