A few fixes to how we check for the sRGB override in the
TextureVk class. In at least one instance there was a potential
edge case where in syncState we might not create the Texture
with the mutable bit the second time through the function.

Bug: angleproject:5176
Change-Id: I4f1ca6e469b10514c3a0de3120be9ade62568084
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2482292Reviewed-by: Mohan Maiya <m.maiya@samsung.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>