drm/mediatek: 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 <javierm@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211217003752.3946210-29-javierm@redhat.com

drivers/gpu/drm/mediatek/mtk_drm_drv.c

@@ -210,6 +210,9 @@ static int mtk_drm_kms_init(struct drm_device *drm)
 	struct device *dma_dev;
 	int ret;
 
+	if (drm_firmware_drivers_only())
+		return -ENODEV;
+
 	if (!iommu_present(&platform_bus_type))
 		return -EPROBE_DEFER;