drm/i915/fbc: Extract intel_fbc_cfb_cpp()

Extract a helper to determine the CFB bytes per pixel value.
Currently this is always 4, but that could change in the
future.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240705145254.3355-14-ville.syrjala@linux.intel.com


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