drm/i915/fbc: Rewrite the FBC tiling check a bit

Write the tiling check in a nicer form. No functional
changes due to Y-tile scanout being a gen9+ feature.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702204603.596-2-ville.syrjala@linux.intel.com