drm/i915: Call the sync_hw hook for power wells without a domain

So far the sync_hw hook wasn't called for power wells not belonging to
any power domain, that is the GEN9 PW1 and MISC_IO power wells. This
wasn't a problem so far since the goal of the sync_hw hook - to clear
the corresponding BIOS request bit - was guaranteed by clearing the
whole BIOS request register elsewhere. This will change with the next
patch, so fix up the inconsistency.

While at it clean up the power well iterator helpers and move them to
the rest of iterators.

v2:
- Clean up the power well iterator helpers. (Ander)
- Move the helpers to i915_drv.h.

Cc: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487345986-26511-3-git-send-email-imre.deak@intel.com