drm/i915: Use drm_modeset_lock_all() in debugfs display info

Make out life easier by just grabbing all modeset locks around
the display_info dump.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191129185434.25549-7-ville.syrjala@linux.intel.com


Reviewed-by: Ramalingam C <ramalingam.c@intel.com>