mesa: Drop _EnabledUnits.

mesa: Drop _EnabledUnits.

The field wasn't really valid, since we've got more than 32 units now.  It
turns out it was mostly just used for checking != 0, or checking for fixed
function coordinates, though.

v2: Fix mis-conversion in xm_line.c (caught by Ken).

Reviewed-by: Matt Turner <mattst88@gmail.com> (v1)
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>