drm/radeon: silence UBSAN warning (v3)

Convert a variable sized array from [1] to [].

v2: fix up a few more.
v3: integrate comments from Kees.

Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Jeff Johnson <quic_jjohnson@quicinc.com> (v2)
Acked-by: Christian König <christian.koenig@amd.com> (v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: keescook@chromium.org