drm/i915: Allow the initial async flip to change modifier

drm/i915: Allow the initial async flip to change modifier

With Xorg+modesetting on skl+ we see the following behaviour:
1. root pixmap is X-tiled
2. client submitted buffers can be Y-tiled (w/ 'Option "dmabuf_capable"')
3. we try to switch from the X-tiled buffer to the Y-tiled buffer
   using an async flip (when vsync is disabled).
4. the async flip will be rejected by i915 due to the modifier change

Relax the rules a bit by turning the first async flip into a sync
flip so that we can change the modifier if necessary. Note that
we already convert the first async flip into a sync flip on adl+
in order to reprogram the watermarks.

Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240430095639.26390-4-ville.syrjala@linux.intel.com