video: fbmem: use const pointer for fb_ops

Use const for fb_ops to let us make the info->fbops pointer const in the
future.

v2: rebase

Cc: linux-fbdev@vger.kernel.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/3a27f95b424a67b3542b5906c660741daf1d4ea6.1575022735.git.jani.nikula@intel.com