drm/exynos: Add support for the nomodeset kernel parameter

According to disable Documentation/admin-guide/kernel-parameters.txt, this
parameter can be used to disable kernel modesetting.

DRM drivers will not perform display-mode changes or accelerated rendering
and only the system framebuffer will be available if it was set-up.

But only a few DRM drivers currently check for nomodeset, make this driver
to also support the command line parameter.

Signed-off-by: Javier Martinez Canillas <[e-mail]>
Acked-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/exynos/exynos_drm_drv.c

···
 {
 	int ret;
 
+	if (drm_firmware_drivers_only())
+		return -ENODEV;
+
 	ret = exynos_drm_register_devices();
 	if (ret)
 		return ret;