drm: constify fb ops across all drivers

Now that the fbops member of struct fb_info is const, we can start
making the ops const as well.

Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/59b43629ac60031c5bbf961d8c49695019bc9c6f.1575390740.git.jani.nikula@intel.com

drivers/gpu/drm/omapdrm/omap_fbdev.c

@@ -70,7 +70,7 @@ fallback:
 	return drm_fb_helper_pan_display(var, fbi);
 }
 
-static struct fb_ops omap_fb_ops = {
+static const struct fb_ops omap_fb_ops = {
 	.owner = THIS_MODULE,
 
 	.fb_check_var	= drm_fb_helper_check_var,