ValidateSamplerParameterBase is called with both const and non-const
parameter pointers which caused our explicit template instantiations to
not cover all cases resulting in link errors.  Force the parameters to
always be const so that our instantiations cover everything.

BUG=angleproject:3555

Change-Id: Ibf92572ba80a689c75dcdc70e5153d9941da76f3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1676305Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>