This won't be used in the future. It saves a few instructions on each
entry point.

Also refactors a bit of touched code. Also adds in a missed entry
point: "glTexStorage2DMultisampleANGLE".

Removes related code and moves remaining helper code in params.h into a
new file entry_point_utils.h.

In total this patch series reduces overhead by up to 5%.

Bug: angleproject:2933
Change-Id: Ifb49564597cde6ba82dfc3e185227619fdc62612
Reviewed-on: https://chromium-review.googlesource.com/c/1299478Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>