drm/xe: Fix media detection for pre-GMD_ID platforms

drm/xe: Fix media detection for pre-GMD_ID platforms

Reading the GMD_ID register on platforms before that register
became available is not reliable. The assumption was that since the
register was not allocated, it would return 0. But on PVC for example it
returns garbage (or a very specific number), triggering the following
error:

	xe 0000:8c:00.0: [drm] *ERROR* Hardware reports unknown media version 1025.55

Fix it by stop relying on the value returned by that registers on
platforms before GMD_ID. Instead this relies on the graphics description
struct being already pre-set on the device: this can only ever be true
for platforms before the GMD_ID support. In that case, GMD_ID is skipped
and the hardcoded values are used.  This should also help on early
bring-up in case the GMD_ID returns something not expected and we need to
temporarily hardcode values. With this, PVC doesn't trigger the error
and goes straight to:

	xe 0000:8c:00.0: [drm:xe_display_info_init [xe]] No display IP, skipping
	xe 0000:8c:00.0: [drm:xe_pci_probe [xe]] XE_PVC  0bd5:002f dgfx:1 gfx:Xe_HPC (12.60) media:none (0.00) dma_m_s:52 tc:2
	xe 0000:8c:00.0: [drm:xe_pci_probe [xe]] Stepping = (G:C0, M:**, D:**, B:B3)

Fixes: 5822bba943ad ("drm/xe: Select graphics/media descriptors from GMD_ID")
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://lore.kernel.org/r/20230427184408.1340988-1-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>