drm/i915/fbc: Extract _intel_fbc_cfb_size()

Pull the lower level stuff out from intel_fbc_cfb_size() into
a separate function that doesn't depend on the plane_state.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240705145254.3355-13-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>