drm/amdkfd: fix null pointer dereference on dev

The pointer dev is set to null yet it is being dereferenced when
checking dev->dqm->sched_policy.  Fix this by performing the check
on dev->dqm->sched_policy after dev has been assigned and null
checked.  Also remove the redundant null assignment to dev.

Addresses-Coverity: ("Explicit null dereference")
Fixes: 1a058c33 ("drm/amdkfd: New IOCTL to allocate queue GWS")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>