amdgpu/util_hash_table: Add helper function to count the number of entries in hash table

Analogous to the mesa commit of the same name.
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>