drm/i915/hsw, bdw: Add an ID for the global display power well

drm/i915/hsw, bdw: Add an ID for the global display power well

Add an ID for the HSW/BDW global display power well for consistency. The
ID is selected so that it can be used to get at the HW request and
status flags with the corresponding GEN9+ macros. Unifying the HSW/BDW
and GEN9+ versions of these macros and the power well ops using them
will be done in follow-up patches.

v2:
- Rebased on v2 of patch 2.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170711204236.5618-3-imre.deak@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>