r300g: cleanup texture creation code

r300g: cleanup texture creation code

This decouples initializing a texture layout/miptree description
from an actual texture creation, it also partially unifies texture_create
and texture_from_handle.

r300_texture inherits r300_texture_desc, which inherits u_resource.

The CBZB clear criteria are moved to r300_texture_desc::cbzb_allowed[level].

And other minor cleanups.