drm/amdgpu: Return -ENOMEM when there is no memory in 'amdgpu_gfx_mqd_sw_init'

Return -ENOMEM, when there is no sufficient dynamically allocated memory
to create MQD backup for ring

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