drm/i915/frontbuffer: Nuke intel_frontbuffer_flip_{prepare,complete}()

drm/i915/frontbuffer: Nuke intel_frontbuffer_flip_{prepare,complete}()

Get rid of intel_frontbuffer_flip_{prepare,complete}() (and
the accompanying flip_bits) since they are unused.

I suppose these could technically provide a minor optimization
over intel_frontbuffer_flip() in that the flush would get
deferred further if new rendering were to sneak in between the
prepare() and complete() calls. But for correctness it should
not make any difference since another flush will anyway follow
once the new rendering finishes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251016185408.22735-4-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>