drm/i915/fbc: Split plane pixel format checks per-platform

Carve up pixel_format_is_valid() into per-platform variants to
make it easier to see what limits are actually being imposed.

Note that the XRGB1555 can be dropped from the g4x+ variant
since the plane no longer supports that format anyway.

TODO: maybe go for vfuncs later

v2: Update for lnl changes

Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com> #v1
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231003194256.28569-5-ville.syrjala@linux.intel.com