[media] vb2: Fix vb2_core_dqbuf() kernel-doc

The kernel-doc has the wrong function name and also the pindex
parameter is missing in the documentation.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>