media: v4l2-ctrls: alloc memory for p_req

To store request data the handler_new_ref() allocates memory
for it if needed.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>