drm/amdkfd: Only load sdma mqd when queue is active

Also calls load_mqd with current->mm struct. The mm
struct is used to read back user wptr of the queue.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>