i915: Add option to bypass vbt table.

This change enables the use of displays where the vbt table just
contains inappropriate values, but either the vesa defaults or
the video=... modes do something sensible with the attached display.

The problem happens with an embedded board that contains vbt bios
tables that do not match the attached display.  Using this change and
the appropriate kernel boot command line they are able to use an
otherwise completely unusable secondary display on that embedded
board.

Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Mathias Froehlich <Mathias.Froehlich@web.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>