drm/i915/frontbuffer: Split fb_tracking.lock into two

drm/i915/frontbuffer: Split fb_tracking.lock into two

Our fb_tracking.lock is serving a double duty:
- protects fb_tracking.busy_bits
- provides the write-side protection for obj->frontbuffer

Split obj->frontbuffer role into a separate lock so that
we can clean up the current mess with the frontbuffer lifetime
management.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251016185408.22735-7-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>