drm/i915/dsi: start using enum mipi_dsi_pixel_format

A small step moving us closer to DRM MIPI DSI code. Use enum
mipi_dsi_pixel_format instead of our own. The first benefit is being
able to use common mipi_dsi_pixel_format_to_bpp().

There's a little back and forth conversion with the VBT -> enum ->
register, since we have just shoved the VBT value into the register
directly. Longer term, all the VBT parsing and deciphering should be
done in intel_bios.c, and abstracted there.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458123700-16003-2-git-send-email-jani.nikula@intel.com