drm/amdgpu: don't clamp debugfs register access to the BAR size

This prevents us from accessing extended registers in tools like
umr.  The register access functions already check if the offset
is beyond the BAR size and use the indirect accessors with locking
so this is safe.

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