drm/xe: Remove gen-based mmio offsets from hw engine init

During early generations of Intel GPUs, hardware engines would sometimes
move to new MMIO offsets from one platform/generation to the next.
These days engines the hardware teams put more effort into ensuring that
engines stay at consistent locations; even major design changes (like
the introduction of standalone media) keep the MMIO locations of the
engines constant.

Since all platforms supported by the Xe driver are new enough to have a
single MMIO offset for each engine (and since our crystal ball says that
these offsets are very unlikely to change again in the foreseeable
future), we can simplify the driver's engine definitions and remove the
gen-based MMIO bases.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>