drm/i915/dp: Return a fixed point BPP value from intel_dp_output_bpp()

drm/i915/dp: Return a fixed point BPP value from intel_dp_output_bpp()

Convert intel_dp_output_bpp() and intel_dp_mode_min_output_bpp() to
return an x16 fixed point bpp value, as this value will be always the
link BPP (either compressed or uncompressed) tracked in the same x16
fixed point format.

While at it rename
intel_dp_output_bpp() to intel_dp_output_format_link_bpp_x16() and
intel_dp_mode_min_output_bpp() to intel_dp_mode_min_link_bpp_x16() to
better reflect that these functions return an x16 link BPP value
specific to a particular output format or mode.

Also rename intel_dp_output_bpp()'s bpp parameter to pipe_bpp, to
clarify which kind of (pipe vs. link) BPP the parameter is.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20251215192357.172201-5-imre.deak@intel.com