drm/bochs: disable video before changing video mode

qemu and simics simulators both seem to expect that video should be disabled
before changing the video mode.

references:
http://git.qemu.org/?p=qemu.git;a=blob;f=hw/display/vga.c;h=c0f7b343bbab586c8593d29c7a765f1e6ca3662c;hb=HEAD#l727
http://wiki.osdev.org/Bochs_VBE_Extensions#Setting_display_resolution_and_bit_depth

Signed-off-by: Chris Ruffin <chris.ruffin@intel.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>