drm/i915/tgl: Set subplatforms

There is no way to differentiate TGL-U from TGL-Y by the PCI ids as
some ids are available in both SKUs.
So here using the root device id in the PCI bus that iGPU is in
to differentiate between U and Y.

BSpec: 44455
Reviewed-by: Swathi Dhanavanthri <swathi.dhanavanthri@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200807192629.64134-1-jose.souza@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>