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

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

Link: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2039926


Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>