drm/imx: dw_hdmi-imx: depend on OF to fix randconfig compile tests on x86_64

drm/imx: dw_hdmi-imx: depend on OF to fix randconfig compile tests on x86_64

The i.MX6 HDMI driver uses of_drm_find_bridge() and thus cannot be built
with CONFIG_OF disabled:

    ld: drivers/gpu/drm/imx/dw_hdmi-imx.o: in function `dw_hdmi_imx_probe':
    dw_hdmi-imx.c:(.text+0x29f): undefined reference to `of_drm_find_bridge'

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Fixes: 743a1bc3669b ("drm/imx: dw_hdmi-imx: move initialization into probe")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>