drm/i915: Fix the async flip wm0/ddb optimization

drm/i915: Fix the async flip wm0/ddb optimization

The current implementation of the async flip wm0/ddb optimization
does not work at all. The biggest problem is that we skip the
whole intel_pipe_update_{start,end}() dance and thus never actually
complete the commit that is trying to do the wm/ddb change.

To fix this we need to move the do_async_flip flag to the crtc
state since we handle commits per-pipe, not per-plane.

Also since all planes can now be included in the first/last
"async flip" (which gets converted to a sync flip to do the
wm/ddb mangling) we need to be more careful when checking if
the plane state is async flip comptatible. Only planes doing
the async flip should be checked and other planes are perfectly
fine not adhereing to any async flip related limitations.

However for subsequent commits which are actually going do the
async flip in hardware we want to make sure no other planes
are in the state. That should never happen assuming we did our
job correctly, so we'll toss in a WARN to make sure we catch
any bugs here.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Fixes: c3639f3be480 ("drm/i915: Use wm0 only during async flips for DG2")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214105532.13049-4-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>