drm/i915/fbc: Use lockdep_assert_held()

drm/i915/fbc: Use lockdep_assert_held()

Replace the mutex_is_locked() stuff with lockdep_assert_held() since
that's what it's there for.

Also sprinkle these around so that we have more or less
mirrored coverage for the enable vs. disable instead of the
current situation where the asserts seem to be more or less
randomly thrown around.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220620182917.10765-2-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>