r300g: raise the number of texture units to 16 for all supported chipsets

r300g: raise the number of texture units to 16 for all supported chipsets

As per Radeon 9700 Opengl Programming and Optimization Guide [1], there are
16 texture units even on the first r300 chipsets. If you think I am wrong,
feel free to propose a patch.

[1] Here's PDF: http://people.freedesktop.org/~mareko/