DiscardFramebuffer was being ignored in the new linear path. Fix this
by using the correct APIs. Also cleans up some of the access to the
"RenderPassCommandBuffer" struct in ContextVk. Add a new accessor that
returns the RenderPass without a flush that ensures it is only called
when a RenderPass is already started.

Bug: angleproject:4029
Change-Id: Ibd199f3a371c0f91b448a0b34c8752dc8b76c284
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2057329Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>