media: mtk-vcodec: vdec: set VPI IPI handler in one place

Each of the supported decoder formats used to set the same RPC
interrupt handler by themselves, even though this could be done
by the IF init function itself. Move it to the right place and stop
making its symbol public.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>