drm/amdgpu/pptable: Fix UBSAN array-index-out-of-bounds

Flexible arrays used [1] instead of []. Replace the former with the latter
to resolve multiple UBSAN warnings observed on boot with a BONAIRE card.

In addition, use the __counted_by attribute where possible to hint the
length of the arrays to the compiler and any sanitizers.

Signed-off-by: Tasos Sahanidis <tasos@tasossah.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>