gallium/radeon: replace is_flushing_texture with db_compatible

gallium/radeon: replace is_flushing_texture with db_compatible

This is a left-over of when I considered generalizing the separate stencil
support. I do prefer the new name since it emphasizes what flushing vs.
non-flushing means from a functional point-of-view, namely special handling
of the texture format.

v2: adjust r600_init_color_surface as well

Reviewed-by: Marek Olšák <marek.olsak@amd.com>