drm/i915/gen9: Disable DC states if power well support is disabled

If power well support is disabled via the i915.disable_power_well module
option we should never enable DC states. Currently we would enable DC
states even in this case during system suspend, where we need to disable
all power wells regardless of the disable_power_well option.

CC: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456778945-5411-3-git-send-email-imre.deak@intel.com