panfrost: allocate bo for occlusion query results

This memory needs to still be available after all the drawing is done and forgotten about, so cannot be transient. Also clear the result so that no rendering returns a zero.

Note: I found this after chasing a segfault in openscad, but since openscad has more trouble with panfrost I'm not actually sure if this makes occlusion queries work right. I'm sure there is a simple enough occlusion query test out there, pointing me to one would be appreciated.