amdgpu/dc: don't check for 0 on register read/writes always.

This adds ~50k to the driver text segment, and 10k to data segment.

   text	   data	    bss	    dec	    hex	filename
2385556	  39681	   1045	2426282	 2505aa	drivers/gpu/drm/amd/amdgpu/amdgpu.o
  text	   data	    bss	    dec	    hex	filename
2336593	  28857	   1045	2366495	 241c1f	drivers/gpu/drm/amd/amdgpu/amdgpu.o

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>