drm/i915: Fix pipe_bpp readout for BXT/GLK DSI

drm/i915: Fix pipe_bpp readout for BXT/GLK DSI

The only bpc information in pipe registers for BXT/GLK DSI
is the PIPEMISC dither bpc. Let's try to use that to read
out pipe_bpp on these platforms. However, I'm not sure if
this will be correctly populated by the GOP since bspec
suggests it's only needed if dithering is actually enabled.
If not I guess we'll have to go one step further and
extract pipe_bpp from the DSI pixel format when dithering
is disabled.

Cc: Hans de Goede <hdegoede@redhat.com>
Fixes: ca0b04db14a5 ("drm/i915/dsi: Fix pipe_bpp for handling for 6 bpc pixel-formats")
References: https://bugs.freedesktop.org/show_bug.cgi?id=109516
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405141349.11950-1-ville.syrjala@linux.intel.com