drm/i915: Convert unconditional clflush to drm_clflush_virt_range()

drm/i915: Convert unconditional clflush to drm_clflush_virt_range()

This one is apparently a "clflush for good measure", so bit more
justification (if you can call it that) than some of the others.
Convert to drm_clflush_virt_range() again so that machines without
clflush will survive the ordeal.

Cc: stable@vger.kernel.org
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Thomas Hellström <thomas.hellstrom@intel.com> #v1
Fixes: 20a7f2ba946d ("drm/i915: Do not share hwsp across contexts any more, v8.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211014090941.12159-3-ville.syrjala@linux.intel.com
Reviewed-by: Dave Airlie <airlied@redhat.com>