Fix memory leak.

For devices without intrinsic fp64 support, a shader-based fallback
is used.  The memory allocated to support this was not being freed.

Change-Id: Id1334c81a2204a6707bc42a78f3a78e60ee3b72f
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/919772
Reviewed-by: Craig Stout <cstout@google.com>
Commit-Queue: Josh Gargus <jjosh@google.com>
Size-Review: Aaron Wood <aaronwood@google.com>