glsl: binding point is a texture unit, which is a combined space

glsl: binding point is a texture unit, which is a combined space

This fixes compilation failures in Dota 2 Reborn where a texture unit
binding point was used that was numerically higher than the max
per stage.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Timothy Arceri <t_arceri@yahoo.com.au>
Tested-by: Nick Sarnie <commendsarnex@gmail.com>
Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org>