drm/radeon: refactor radeon_atif_call

Don't hard-code function number, this will allow to reuse the function.
v2: add support for the 2nd parameter (from Lee, Chun-Yi
<jlee@suse.com>).

Signed-off-by: Luca Tettamanti <kronos.it@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>