drm/i915: Release connector iterator on a digital port conflict.

Hitting the failure path through check_digital_port_conflicts triggers:

================================================
WARNING: lock held when returning to user space!
4.16.0-rc1-CI-kasan_1+ #1 Tainted: G        W
------------------------------------------------
kms_3d/1439 is leaving the kernel with locks still held!
1 lock held by kms_3d/1439:
 #0:  (drm_connector_list_iter){.+.+}, at: [<000000003745d183>] intel_atomic_check+0x1d9d/0x3ff0 [i915]

Rearrange the code to have a single exit path through the unlock.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180215091425.42364-1-maarten.lankhorst@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>