staging: gma500: Skip bogus LVDS VBT mode and check for LVDS before adding backlight

On the Fit-PC2 the VBT reports an invalid fixed panel mode for LVDS, this gets
in the way for SDVO. This patch makes VBT parsing skip the invalid mode. When
there is no LVDS output the backlight support crashes so the patch also checks
for this before enabling it.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>