drm/i915: Fix power domain HW state cleanup on error path

Move the cleanup of the power domain HW state on the error path to the
same function where the corresponding init call was called from. I
noticed this problem when loading the module with load failure injection
enabled, making i915_load_modeset_init() fail.

CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1458128348-15730-19-git-send-email-imre.deak@intel.com