drm/i915: Move the async_flip bit setup into the .async_flip() hook

drm/i915: Move the async_flip bit setup into the .async_flip() hook

Set up the async flip PLANE_CTL bit directly in the
.async_flip() hook. Neither .update_plane() nor .disable_plane()
ever need to set this so having it done by skl_plane_ctl_crtc()
is rather pointless.

Cc: Karthik B S <karthik.b.s@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111163711.12913-7-ville.syrjala@linux.intel.com
Reviewed-by: Karthik B S <karthik.b.s@intel.com>