When the source format is 10_10_10_2, valueBits needs to be
"2" if the component is "3" i.e, the alpha channel. It was
incorrectly set to "10" for all components

Bug: angleproject:3866
Change-Id: If206c63d704a7a485681e483e12ae9ab611b0d99
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1775105Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>