drm/i915: Merge TARGET_POWER_ON and PANEL_POWER_ON flag definitions

These two flags mean the same thing, so remove the duplication.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470827254-21954-2-git-send-email-imre.deak@intel.com