drm/i915/fbc: Skip nuke when flip is pending

drm/i915/fbc: Skip nuke when flip is pending

Don't issue a nuke from frontbuffer flush while a flip is pending.
This avoids the DSPADDR/DSPSURF rmw abuse from the pre-snb nuke
from racing with the DSPADDR/DSPSURF write being performed by
the flip/plane update. The flip itself will already cause the nuke
so a double nuke is redundant.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315140001.1172-4-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>