Move the flip_pending assignment into __intel_fbc_post_update()
from intel_fbc_post_update(). Now mirrors the pre_update() side.

The only reason the assignment was in the higher level function
is that we used to call __intel_fbc_post_update() from elsewhere
as well. That got cleaned up in commit b39d2c62 ("drm/i915/fbc:
Call intel_fbc_activate() directly from frontbuffer flush")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220620182917.10765-1-ville.syrjala@linux.intel.comReviewed-by: Mika Kahola <mika.kahola@intel.com>