drm/i915: Preserve VGACNTR bits from the BIOS

My Fujistsu-Siemens Lifebook S6010 doesn't like to resume from
S3 unless VGACNTR has been restore to the original value. The BIOS
value in this case was 0x0124008E. Setting the "VGA disable" bit
doesn't interfere with the S3 resume fortunately.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Thomas Richter <richter@rus.uni-stuttgart.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>