drm/amdgpu/sdma4: use a helper for SDMA_OP_POLL_REGMEM

Rather than opencoding it in a bunch of functions.

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>