drm/amd: fix the warning unchecking return vaule for sdma_v7

check ring allocate success before emit preempt ib

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>