drm/i915/dsi: use the BIT macro for clarity

No functional changes.

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/9115c0a80ad57075700e006db965dd31cc4358fc.1458313400.git.jani.nikula@intel.com