drm/i915: pass mode to ELD write vfuncs

This will be needed for setting the HDMI pixel clock for audio
config. No functional changes.

v2: Now with a commit message.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>