media: mtk-vcodec: avoid unneeded pointer-to-long conversions

The interface used to communicate with the firmware casts pointers
into unsigned longs and back again in order to store private
references, all of this for pointers that remain purely in the kernel.
Replace these unsigned longs with void pointers to make the code a bit
sturdier and easier to follow.

Also simplify some interfaces by removing arguments that could be
infered from others.

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