drm/amdgpu/userq: rework front end call sequence

Split out the queue map from the mqd create call and split
out the queue unmap from the mqd destroy call.  This splits
the queue setup and teardown with the actual enablement
in the firmware.

Reviewed-by: Sunil Khatri <sunil.khatri@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>