Make the DRM-format API leaner

This addresses the following comments from @afrantzis : !544 (comment 960202)

... weston_drm_format_array_intersect() returns a new array leaving the sources unaffected, whereas the other set functions (join, subtract) operate on the first operand. I guess this is driven by the particular needs of the code in scanout_formats_add_planes_formats, but the end result is an API with reduced consistency.

... The intersection function returning a new object is also the only real reason to expose the weston_drm_format_array_destroy() function. From what I see, other uses of create/destroy can be trivially replaced by init/fini.