drm/i915/gen2: Add an ID for the display pipes power well

drm/i915/gen2: Add an ID for the display pipes power well

Make the I830 power well ID assignment explicit for consistency.

v2:
- s/GEN2/I830/ in the comment, since other GEN2s don't have the power
  well. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170711204236.5618-2-imre.deak@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>