drm/amdgpu: fix error checking in amdgpu_read_mm_registers for nv

Properly skip non-existent registers as well.

Bug: drm/amd#2442


Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org