drm/i915: Clean up the baseline bpp computation

drm/i915: Clean up the baseline bpp computation

Pass on the errno all the way from connected_sink_max_bpp(),
and make the base_bpp handling in intel_modeset_pipe_config()
a bit less ugly. We'll also rename connected_sink_max_bpp()
to not give the impression that it return the bpp value,
and we'll pimp up the debug message within to include the
connector name/id.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181107213522.17590-3-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>