staging: mmal-vchiq: Replace spinlock protecting context_map with mutex

950fd867 staging: bcm2835-camera: Replace open-coded idr with a struct idr.
replaced an internal implementation of an idr with the standard functions
and a spinlock.
idr_alloc(GFP_KERNEL) can sleep whilst calling kmem_cache_alloc to allocate
the new node, but this is not valid whilst in an atomic context due to the
spinlock.

There is no need for this to be a spinlock as a standard mutex is
sufficient.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>