In https://chromium-review.googlesource.com/c/453888/, we changed how
blendstates were applied in masked clears. This change would apply a
null blendstate when all color channels were disabled, but this seems
to have an issue on Intel Cherry View drivers. Work around this issue
by restoring the prior functionality of making a simple masked blend
state.

Also clean up some of the code style in the Clear11 class.

BUG=chromium:730126

Change-Id: I9a4044201b2f07e9483525513a59e19bb2a8bcd3
Reviewed-on: https://chromium-review.googlesource.com/533684Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>