drm/xe: Drop unnecessary STOP_RING clearing

drm/xe: Drop unnecessary STOP_RING clearing

The STOP_RING bit in MI_MODE is already clear by default out of hardware
reset and will only be '1' if the driver intentionally sets it after
that.

The logic of clearing this bit appears to originate from very
early (pre-GuC, pre-execlist) code in i915 where we needed to stop the
ring before performing a host-initiated engine reset; after the reset
the STOP_RING bit needed to be cleared to allow execution to resume.

None of that is relevant to Xe (or even modern i915) since STOP_RING
isn't necessary for execlist-based engine resets (and even if it were,
Xe doesn't initiate any engine resets; the GuC handles that now).

Bspec: 60356, 60184
Reviewed-by: Shuicheng Lin <shuicheng.lin@intel.com>
Link: https://patch.msgid.link/20260424-engine-setup-v2-8-59cc620a25f1@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>