drm/i915/fbc: Call intel_fbc_activate() directly from frontbuffer flush

Remove the pointless detour via __intel_fbc_post_update() during
frontbuffer flush.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315140001.1172-8-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>