drm/xe/irq: Convert register access to use xe_mmio

drm/xe/irq: Convert register access to use xe_mmio

Stop using GT pointers for register access.  This misusage has been
especially confusing in interrupt code because even though some of the
interrupts are related to GTs (or engines within GTs), the interrupt
registers themselves live outside the GT, in the sgunit.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240910234719.3335472-55-matthew.d.roper@intel.com