drm/amdgpu: add sw_init to df_v1_7

change df_init to df_sw_init df 1.7 to prevent regression issues on pre-vega20
products when callback is called in sw_common_sw_init.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Jonathan Kim <Jonathan.Kim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>