drm/i915: Reject async flips if we need to change DDB/watermarks

drm/i915: Reject async flips if we need to change DDB/watermarks

DDB/watermarks are always double buffered on the vblank, so we
can't safely change them during async flips. Currently this never
happens, but we'll be making changing between sync and async
flips a bit more flexible, in which case we can actually end up
here.

Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240430095639.26390-3-ville.syrjala@linux.intel.com