drm/i915/dp: Update the link bpp limits for DSC mode

In non-DSC mode the link bpp can be set in 2*3 bpp steps in the pipe bpp
range, while in DSC mode it can be set in 1/16 bpp steps to any value
up to the maximum pipe bpp. Update the limits accordingly in both modes
to prepare for a follow-up patch which may need to reduce the max link
bpp value and starts to check the link bpp limits in DSC mode as well.

While at it add more detail to the link limit debug print and print it
also for DSC mode.

v2:
- Add to_bpp_frac_dec() instead of open coding it. (Jani)
v3: (Ville)
- Add BPP_X16_FMT / BPP_X16_ARG.
- Add TODO: comment about initializing the DSC link bpp limits earlier.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230921195159.2646027-5-imre.deak@intel.com