drm/msm: Add NULL check in vm_op_enqueue()

drm/msm: Add NULL check in vm_op_enqueue()

vm_op_enqueue() allocates an msm_vm_op struct with kmalloc,
but the return value is not checked for NULL value which
can be returned by kmalloc under low-memory conditions.
This can result in NULL pointer dereference when the pointer
is dereferenced.

Add NULL check after the allocation and propagate -ENOMEM back
to the caller in case of a failure.

Signed-off-by: Gopi Krishna Menon <krishnagopi487@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/678416/
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>