drm/i915/dsi: set pipe_bpp on ICL configure config

The ICL DSI pipe_bpp currently comes from
compute_baseline_pipe_bpp(). Fix it.

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/191f5c4fa5f4af29d4bf7e30bb35f45ce05b33f0.1575974743.git.jani.nikula@intel.com