gallium: clean up resource_copy_region function

gallium: clean up resource_copy_region function

Previously, surface_copy was said to allow overlapping blits, and it was
"optional". However, some state trackers actually assumed it is always present,
and quite some code (like in u_blit) assumed overlapping isn't allowed.
Hence, resource_copy_region (and in the same spirit, resource_fill_region) is
now mandatory, but overlapping blits are no longer allowed. A driver can plug
in the cpu fallback util_resource_copy_region if it does not want to provide its
own implementation, though this is not optimal.