drm/i915: make the intel_display_power_domain enum compact

Upcoming patches will add tracking for a set of power domains via a
bitmask; to make things simple there remove the current gap in the
enum values.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>