__i915_gem_object_flush_map() takes a byte range, so feed it the written
bytes and do not mistake the u32 index as bytes!

Fixes: a679f58d ("drm/i915: Flush pages on acquisition")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: <stable@vger.kernel.org> # v5.2+
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200406114821.10949-1-chris@chris-wilson.co.uk