drm/amdgpu: add debugfs interface for reading MQDs

Provide a debugfs interface to access the MQD.  Useful for
debugging issues with the CP and MES hardware scheduler.

v2: fix missing unreserve/unmap when pos >= size (Alex)

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>