[media] media: vb2: review mem_priv usage and fix potential bugs

This patch is a result of review of mem_priv entry usage in videobuf2 core.
It fixes all all potential places where it was not checked against NULL or
zeroed after freeing as well as a few style issues.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
CC: Pawel Osciak <pawel@osciak.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>