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>

drivers/gpu/drm/bochs/bochs_hw.c

@@ -164,6 +164,7 @@ void bochs_hw_setmode(struct bochs_device *bochs,
 
 	bochs_vga_writeb(bochs, 0x3c0, 0x20); /* unblank */
 
+	bochs_dispi_write(bochs, VBE_DISPI_INDEX_ENABLE,      0);
 	bochs_dispi_write(bochs, VBE_DISPI_INDEX_BPP,         bochs->bpp);
 	bochs_dispi_write(bochs, VBE_DISPI_INDEX_XRES,        bochs->xres);
 	bochs_dispi_write(bochs, VBE_DISPI_INDEX_YRES,        bochs->yres);