drm/i915: Drop wmb() inside pread_gtt

Inside pread, we only ever read from the GTT so the serialising wmb()
instructions around the GGTT PTE updates are pointless.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718145407.21352-1-chris@chris-wilson.co.uk