drm/amdgpu/atomfirmware: use proper index for querying vram type (v3)

The index is stored in scratch register 4 after asic init.  Use
that index.  No functional change since all asics in a family
use the same type of vram (G5, G6, HBM) and that is all we use
at the monent, but if we ever need to query other info, we will
now have the proper index.

v2: module array is variable sized, handle that.
v3: fix off by one in array handling

Reviewed-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>