drm/xe/oa: Fix static checker warning about null gt

drm/xe/oa: Fix static checker warning about null gt

There is a static checker warning that gt returned by xe_device_get_gt can
be NULL and that is being dereferenced. Use xe_root_mmio_gt instead, which
is equivalent and cannot return a NULL gt 0.

Fixes: 10d42ef34bce ("drm/xe/oa: Assign hwe for OAM_SAG")
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://lore.kernel.org/r/20250715181422.2807624-1-ashutosh.dixit@intel.com