drm/i915/debugfs: Add panel delays for eDP

drm/i915/debugfs: Add panel delays for eDP

The eDP backlight and panel enable/disable delays are quite
useful to know when measuring time consumed by suspend/resume,
and while the information is printed to the kernel log as debug
messages, having this information in debugfs makes things easier.

Signed-off-by: David Weinehall <david.weinehall@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160823092356.7610-1-david.weinehall@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>