drm/omap: 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/20211217003752.3946210-31-javierm@redhat.com

drivers/gpu/drm/omapdrm/omap_drv.c

@@ -727,6 +727,9 @@ static int omapdrm_init(struct omap_drm_private *priv, struct device *dev)
 
 	DBG("%s", dev_name(dev));
 
+	if (drm_firmware_drivers_only())
+		return -ENODEV;
+
 	/* Allocate and initialize the DRM device. */
 	ddev = drm_dev_alloc(&omap_drm_driver, dev);
 	if (IS_ERR(ddev))