gma500: medfield: fix build without CONFIG_BACKLIGHT_CLASS_DEVICE

drivers/built-in.o: In function `mdfld_dsi_connector_set_property':
mdfld_dsi_output.c:(.text+0x6e909): undefined reference to `mdfld_set_brightness'
make: *** [.tmp_vmlinux1] Error 1

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>