nv50,nvc0: bump minimum texture buffer offset alignment

nv50,nvc0: bump minimum texture buffer offset alignment

It appears that it actually needs to be aligned to the datum size, so it
was 1 when testing with R8, but it can be as high as 16 with RGBA32.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>