drm/i915: Intel panel detection cleanup

Let's switch to use private dev_priv instead of dev when detecting
intel panels.

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481616169-16210-2-git-send-email-mika.kahola@intel.com