drm/i915: Replace to_bpp_frac() with fxp_q4_to_frac()

Replace the to_bpp_frac() helper defined by the driver with the
equivalent fxp_q4_to_frac() helper defined by DRM core.

v2: Rebase on the s/drm_x16/fxp_q4 change.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240805150802.3568970-5-imre.deak@intel.com