[ozone/x11] Enabled fallback to SkiaOutputDeviceX11.

SkiaOutputSurfaceImplOnGpu::InitializeForVulkan() had a fallback
implementation to use on X11, namely SkiaOutputDeviceX11, which was not
available for Ozone/X11.

This CL fixes that, and also removes the possibility to create
SkiaOutputDeviceVulkan when vulkan surface is explicitly disabled via
GPU preferences.

Bug: 1121918
Change-Id: I8efe5b01adef97eaade1b101c41c10d509e9cef5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2658317


Commit-Queue: Alexander Dunaev <adunaev@igalia.com>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#849096}