media: 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.

Remove the redundant fbops assignments while at it.

v2:
- actually add const in vivid
- fix typo (Christophe de Dinechin)

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Andy Walls <awalls@md.metrocast.net>
Cc: linux-media@vger.kernel.org
Cc: ivtv-devel@ivtvdriver.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/71794337f8611271f2c1fdb3882119a58e743a87.1575390741.git.jani.nikula@intel.com