drm/vc4: Drop struct_mutex around CL validation.

We were using it so that we could make sure that shader validation
state didn't change while we were validating, but now shader
validation state is immutable.  The bcl/rcl generation doesn't do any
other BO dereferencing, and seems to have no other global state
dependency not covered by job_lock / bo_lock.  We only need to hold
struct_mutex for object unreferencing.

Fixes a lock order reversal between mmap_sem and struct_mutex.

Signed-off-by: Eric Anholt <eric@anholt.net>