drm/i915: Override SDVO panel type in VBT

Judging by comments in the BIOS, if the SDVO LVDS option h40 is enabled,
then we are supposed to query the real panel type via Int15. We don't do
this and so for the Sony Vaio VGC-JS210J which has otherwise default
values, we choose the wrong mode.

This patch adds a driver option, i915.vbt_sdvo_panel_type, which can be
used to override the value in the VBT.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=33691
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>