drm/i915/fbc: Precompute gen9 cfb stride w/a

Precompute the override cfb stride value so that we can check
it when determining if flip nuke can be used or not.

The hardware has 13 bits for this, so we can shrink the storage
to u16 while at it.

v2: Don't explode when crtc_state->enable_fbc lies to us

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127201222.16669-6-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>