drm/xe/mmio: Use struct xe_reg

drm/xe/mmio: Use struct xe_reg

Convert all the callers to deal with xe_mmio_*() using struct xe_reg
instead of plain u32. In a few places there was also a rename
s/reg/reg_val/ when dealing with the value returned so it doesn't get
mixed up with the register address.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20230508225322.2692066-2-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>