drm/i915: use a substruct in vbt data for edp

Housekeeping, similar to psr, backlight, and dsi. No functional changes.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458834623-8734-2-git-send-email-jani.nikula@intel.com