drm/amdkfd: Make SDMA engine number an ASIC-dependent variable

On Raven there is only one SDMA engine instead of previously assumed two,
so we need to adapt our code to this new scenario.

Signed-off-by: Yong Zhao <yong.zhao@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>