drm/i915: Enum forcewake domains and domain identifiers

Make the domains and domain identifiers enums. To emphasize
the difference in order to avoid mistakes.

v2: s/fw_domain/forcewake_domain (Jani)
v3: rebase

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>