drm/amdgpu: simplify nv and soc21 read_register functions

Check of the base offset for the IP exists rather than
explicitly checking for how many instances of a particular
IP there are.  This is what soc15.c already does.  Expand
this to nv.c and soc21.c.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>