[graphite] Check for Vulkan depth-stencil support before
initSampleCounts.

Fixes a crash when trying to run initSampleCounts on an invalid format.

Change-Id: Ia05dda6c5dc0c9f91a86be57a8bed46b984261bb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/705986
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
1 file changed