Remove unnecessary RGB565 conversion.

Bug 21716622

Change-Id: I60a90b91371276f512befb9c1b809a3c4d388eb2
Reviewed-on: https://swiftshader-review.googlesource.com/3447
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>