drm/i915/bxt: add missing DSI power domain to power well 1

The DSI power domain was missing from BXT power well 1 definitions,
failing to get the power well for DSI transcoders. As pipe A is in the
same power well as DSI transcoders, the problem should only occur with
pipes B and C.

According to Ville, this is basically a nop since pw1 is under dmc
control. But given that we still have this stuff defined here, it's
clearly correct to include DSI here.

Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Deepak M <m.deepak@intel.com>
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/1457463656-29357-1-git-send-email-jani.nikula@intel.com