V4L/DVB (12178): vpif_display: Fix compile time warnings for mutex locking

mutex_lock_interruptible return value has to be handled properly to indicate
the status to the higher layers of the kernel.

Signed-off-by: Chaithrika U S <chaithrika@ti.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>