drm/radeon: cleanup coding style a bit

Maybe no need to check ws before kmalloc, kmalloc will check
itself, kmalloc`s logic is if ptr is NULL, kmalloc will just
return

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>