drm/i915/gen9+: Set same power state before hibernation image save/restore

Atm, on GEN9 big core platforms before saving the hibernation image we
uninitialize the display, disabling power wells manually, while before
restoring the image we keep things powered (letting HW/DMC power down
things as needed). The state mismatch will trigger the following error:

DC state mismatch (0x0 -> 0x2)

While the restore handler knows how to initialize the display from an
unknown state (due to a different loader kernel or not having i915
loaded in the loader kernel) we should still use the same state for
consistency before image saving and restoring. Do this by uniniting the
display before restoring the image too.

Bugzilla: https://bugs.freedesktop.org/attachment.cgi?id=133376
Reported-and-tested-by: Wang Wendy <wendy.wang@intel.com>
Reported-and-tested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Wang Wendy <wendy.wang@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170816144607.9935-1-imre.deak@intel.com