drm/xe: Select graphics/media descriptors from GMD_ID

If graphics_desc and media_desc are not specified in a platform's
xe_device_desc, treat this as an indication that the IP version should
be determined from the hardware's GMD_ID register.

Note that leaving media_desc unset for a platform that simply doesn't
have the IP (e.g., PVC) is also okay --- a read of the GMD_ID register
offset will be attempted, but since there's no register at that location
a value of '0' will be returned, effectively disabling media support.

Mapping of version -> IP description is done via a table lookup; this
table will be re-used in future patches for some KUnit testing.

v2:
 - Drop dummy structures.  NULL can be safely used for both the GMD_ID
   cases and the "media not present case."
 - Use a table-based lookup of GMD_ID versions rather than a simple
   switch statement; the table will allow us to easily perform kunit
   testing of all the IP descriptors.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20230406235621.1914492-8-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

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