drm/xe: Move IRQ-related registers to dedicated header

IRQ registers have a well-defined scope and make sense to collect in a
dedicated header file.  This also reduces confusion about the GT IRQ
registers --- even though those registers relate to the GTs, they
actually live outside the GT (in the sgunit) and thus do not need to
worry about GT-specific register concepts like forcewake, steering, etc.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240923214514.2031410-2-matthew.d.roper@intel.com