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