drm/amdgpu: convert some variable sized arrays to [] style

Replace [1] with [].  Silences UBSAN warnings.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3107


Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>