drm/amdgpu: Off by one sanity checks

This is just future proofing code, not something that can be triggered
in real life.  We're testing to make sure we don't shift wrap when we
do "1ull << i" so "i" has to be in the 0-63 range.  If it's 64 then we
have gone too far.

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