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 <[e-mail]>
Acked-by: Randy Dunlap <[e-mail]> # build-tested
Fixes: c805ec7eb210 ("drm/imx: dw_hdmi-imx: move initialization into probe")
Signed-off-by: Philipp Zabel <[e-mail]>

drivers/gpu/drm/imx/Kconfig

···
 config DRM_IMX_HDMI
 	tristate "Freescale i.MX DRM HDMI"
 	select DRM_DW_HDMI
-	depends on DRM_IMX
+	depends on DRM_IMX && OF
 	help
 	  Choose this if you want to use HDMI on i.MX6.