drm/xe: Export xe_hw_engine's mmio accessors

drm/xe: Export xe_hw_engine's mmio accessors

Export hw engine's mmio accessors. This is in preparation
to use these from eudebug code.

v2: s/hw_engine_mmio/xe_hw_engine_mmio (Matthew)
v3: kernel doc (Matthew)

Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Dominik Grzegorzek <dominik.grzegorzek@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240806153009.1081382-1-mika.kuoppala@linux.intel.com