drm/i915: Handle -EDEADLK in drm_atomic_commit from load-detect.

drm/i915: Handle -EDEADLK in drm_atomic_commit from load-detect.

CI runs with DEBUG_WW_MUTEX_SLOWPATH, so -EDEADLK occurs a lot more.
Handle the case where drm_atomic_commit fails with -EDEADLK correctly.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/56D3FEF1.6070306@linux.intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
(cherry picked from commit 3ba86073edcbe2be53d9862d5a3098f0ebf8ae9a)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>