gallivm: fix crash with seamless cube filtering with different min/mag filter

gallivm: fix crash with seamless cube filtering with different min/mag filter

We are not allowed to modify the incoming coords values, or things may
crash (as we may be inside a llvm conditional and the values may be used
in another branch).
I recently broke this when fixing an issue with NaNs and seamless cube
map filtering, and it causes crashes when doing cubemap filtering
if the min and mag filters are different.
Add const to the pointers passed in to prevent this mishap in the future.

Fixes: a485ad0bcd ("gallivm: fix an issue with NaNs with seamless cube filtering")
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>