drm/xe: Drop xe_hw_engine_mmio_write32()

drm/xe: Drop xe_hw_engine_mmio_write32()

xe_hw_engine_mmio_write32() is only used in a single place and is easily
replaced by a regular xe_mmio_write32() call.  Register read/write
interfaces are already complicated enough with MCR vs non-MCR handling,
so we should avoid adding extra wrappers that just make it more
confusing what to use.

xe_hw_engine_mmio_write32() did have a forcewake assertion that we're
dropping here, but that assertion wasn't entirely correct anyway.  It was
checking hwe->domain which is currently set to XE_FW_RENDER for the BCS
engine, even though BCS engines reside in the GT domain.

v2:
 - Drop prototype in header file as well.  (Shuicheng)

Cc: Shuicheng Lin <shuicheng.lin@intel.com>
Reviewed-by: Shuicheng Lin <shuicheng.lin@intel.com>
Link: https://patch.msgid.link/20260424-engine-setup-v2-9-59cc620a25f1@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>