i965: Remove clearing of bo->map_gtt after failure

i965: Remove clearing of bo->map_gtt after failure

With the conversion to storing the result of drm_mmap to a local and not
directly to bo->map_gtt itself, we no longer should clear bo->map_gtt.
In the best the operation is redundant as we know bo->map_gtt to already
be NULL, but in the worst case we overwrite a concurrent thread that
successfully mmaped the GTT.

Fixes: 314647c4c206 ("i965: Drop global bufmgr lock from brw_bo_map_* functions.")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>