media: rockchip/vpu: Rename rockchip_vpu_common.h into rockchip_vpu_v4l2.h

We're about to add prototypes for the vb2/v4l2 helpers shared by the
encoder/decoder logic in this file, so let's pick a name that reflects
that (rockchip_vpu_common.h was a bit to generic).

Suggested-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>