r300g: finally fix the texture corruption on r3xx-r4xx

r300g: finally fix the texture corruption on r3xx-r4xx

Even though a bound texture stays bound when calling set_fragment_sampler_views,
it must be assigned a new cache region depending on the occupancy of other
texture units.

This fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=28800

Thanks to Álmos <aaalmosss@gmail.com> for finding the bug in the code.

NOTE: This is a candidate for both the 7.9 and 7.10 branches.