mesa: fix texture3D mipmap generation for UNSIGNED_BYTE_3_3_2 and 4_4

Oops, I copy-pasted a typo from 3_3_2.

The 3_3_2 part is a candidate for 7.9 and 7.10.
The 4_4 part isn't, because AL44 is in neither branches.
1 file changed