compositor-drm: Add explicit type member to drm_fb

compositor-drm: Add explicit type member to drm_fb

Rather than magically trying to infer what the buffer is and what we
should do with it when we go to destroy it, add an explicit type
instead.

In doing so, the test for dumb images (destroying them, but only if
they're not the 'live' ones) is removed. This was dead code, as the only
path which could cause us to shuffle images is drm_output_switch_mode.
This calls drm_output_release_fb before the images are reallocated in
drm_output_fini_pixman / drm_output_init_pixman, with the reallocation
unconditionally destroying the images, so can never be hit.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>