media: davinci: vpbe_osd: mark read reg function as __always_unused

Such function is currently unused, but could be needed in the
future. So, keep it, marking it as __always_unused, in order to
avoid a clang W=1 error.

Reviewed-by: Lad Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>