drm/xe/kunit: Add stub to read_gmdid

drm/xe/kunit: Add stub to read_gmdid

Currently it's not possible to test the WAs for platforms using gmdid
since they don't have the IP information on the descriptor struct. In
order to allow that, add a stub function for read_gmdid() that is
activated when the test executes, replacing the iomap and read of the
real register.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20231129232807.1499826-5-lucas.demarchi@intel.com
Link: https://lore.kernel.org/r/20231205133954.2089546-3-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>