mesa: Skip redundant texture completeness checking during image validation.

mesa: Skip redundant texture completeness checking during image validation.

The call to _mesa_test_texobj_completeness() is unnecessary if the
texture is already known to be complete.  If the texture object is
dirtied in the meantime _BaseComplete and _MipmapComplete will be
reset to false.  _mesa_is_image_unit_valid() will start to be called
more frequently in a future commit, so it seems desirable to avoid the
unnecessary work.

Tested-by: Ye Tian <yex.tian@intel.com>
CC: "11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>