drm/amdgpu: add emit_reg_write_reg_wait ring callback

This callback writes a value to a register and then reads
back another register and waits for a value in a single
operation.

Provide a helper function using two operations for engines
that don't support this opertion.

Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>