amdgpu: add env support for amdgpu.ids path

In some cases, like when building a Snap application that uses libdrm, the amdgpu.ids file isn't directly available at the compiling place, but inside a mounted folder. This forces each application to link/bind the file from the current place (usually at $SNAP/gnome-platform/usr/share/libdrm/amdgpu.ids) into the original path (usually at /usr/share/libdrm/amdgpu.ids), which is cumbersome.

This patch allows to set an environment variable, called AMDGPU_ASIC_ID_TABLE_PATH, where the file will be searched for instead of the default, meson-configured, path.