drm/i915: Replace BPP_X16_FMT()/ARGS() with FXP_Q4_FMT()/ARGS()

Replace the BPP_X16_FMT()/ARGS() helpers defined by the driver with the
equivalent FXP_Q4_FMT()/ARGS() helpers 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/20240805150802.3568970-6-imre.deak@intel.com