drm/i915/dp: Use consistent name for link bpp and compressed bpp

Currently there are many places where we use output_bpp for link bpp and
compressed bpp.
Lets use consistent naming:
output_bpp : The intermediate value taking into account the
output_format chroma subsampling.
compressed_bpp : target bpp for the DSC encoder.
link_bpp : final bpp used in the link.

For 444 sampling without DSC:
link_bpp = output_bpp = pipe_bpp

For 420 sampling without DSC:
output_bpp = pipe_bpp / 2
link_bpp = output_bpp

For 444 sampling with DSC:
output_bpp = pipe_bpp
link_bpp = compressed_bpp, computed with output_bpp (i.e. pipe_bpp in
this case)

For 420 sampling with DSC:
output_bpp = pipe_bpp/2
link_bpp = compressed_bpp, computed with output_bpp

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230817142459.89764-5-ankit.k.nautiyal@intel.com