drm/client: Remove drm_client_framebuffer_delete()

drm/client: Remove drm_client_framebuffer_delete()

Release client buffers with drm_client_buffer_delete() instead of
drm_client_framebuffer_delete(). The latter is just a tiny wrapper
around the former.

Move the test for !buffer into drm_client_buffer_delete(), although
all callers appear to always have a valid pointer.

v2:
- test for !buffer before deref-ing pointer (Jocelyn, Dan)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Francesco Valla <francesco@valla.it>
Link: https://patch.msgid.link/20251027121042.143588-6-tzimmermann@suse.de