drm/i915: Shrink bxt_ddi_buf_trans

All the values we put into the BXT buf_trans tables fit into 8 bits.
So switch over to u8 from the u32 we use currently.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170918182604.9519-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>