drm/i915: Make power well ID names more uniform

The format for the ID names is <platform>_DISP_PW_* so rename the IDs
not following this accordingly. Leave BXT_DPIO_CMN_BC as-is since we'll
change that to use another existing ID in the next patch.

v2:
- Fix line over 80 chars checkpatch warning.

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180806095843.13294-9-imre.deak@intel.com