mesa: skip _MaxElement computation unless driver needs strict bounds checking

mesa: skip _MaxElement computation unless driver needs strict bounds checking

If Const.CheckArrayBounds is false, the only code using _MaxElement is
glDrawRangeElements, so I changed it and explained in the code why
_MaxElement is not very useful there.

BTW, the big magic number was copied to the letter
from _mesa_update_array_max_element.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>