drm/xe/oa: Use explicit struct initialization for struct xe_oa_regs

drm/xe/oa: Use explicit struct initialization for struct xe_oa_regs

Use explicit struct initialization for struct xe_oa_regs to reduce chance
of error. Also add .oa_mmio_trg field to struct for completeness.

Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patch.msgid.link/20251202025115.373546-2-ashutosh.dixit@intel.com