drm/amdgpu: fixed raven psp cmd prepare and submit

- fw_size in psp_v10_0_prep_cmd_buf is wrongly set as 0
- fixed the wrong calculation of psp_write_ptr_reg in psp_v10_0_cmd_submit

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>