drm/radeon: fix atom aux payload size check for writes (v2)

The atom aux param interface only supports 4 bits for
the total write transfer size (header + payload).  This
limits us to 12 bytes of payload rather than 16.  Add a
check for this. Reads are not affected.

v2: switch to WARN_ON_ONCE

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>