[media] vb2: Merge vb2_internal_qbuf and vb2_qbuf

After all the code refactoring, vb2_internal_qbuf is only called by
vb2_qbuf.

Since the function it is very simple, there is no need to have
two functions.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>