drm/amd: Fix UBSAN array-index-out-of-bounds for Powerplay headers

For pptable structs that use flexible array sizes, use flexible arrays.

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


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