mesa/st: fix regression for basic drivers

Drivers that doesn't support conditional rendering can't really require respecting conditional rendering, so let's just

This fixes a problem where util_can_blit_via_copy_region started unconditionally rejecting all blits that originate from glBlitFramebuffer, even for drivers where this can't possibly be a problem.

Fixes: 767f70df ("gallium/util: fix util_can_blit_via_copy_region for conditional rendering")