drm/radeon: Implement SDMA interface functions

This patch implements the new SDMA interface functions. It also adds defines
and structures related to SDMA registers.

v2: Removed init_sdma_engines() from interface. Initialization is done in
radeon.

v3:
- Removed unused defines.
- Added SDMA_ prefix to defines that didn't have them.

Signed-off-by: Ben Goz <ben.goz@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>