Fix the handling of CodeCache return codes in pixelflinger

The error condition was not correctly detected: an error was
reported by checking that the index returned by CodeCache::cache
was non-zero. This did not work because this function can return
a positive value on success.

Change-Id: I1f90125ee62ab277b80da4dfb341733cd6e8e86a
Signed-off-by: Kévin PETIT <kevin.petit@arm.com>