[graphite] Cache single-texture BindGroups on DawnTexture

* This follows the Vulkan backend in caching single-texture bind groups on to backend Texture implementations. This allows us to remove DawnResourceProvider's special caching of single-texture bindgroups and findOrCreateSingleTextureSamplerBindGroup(...).

* The DawnCommandBuffer instead defines the bind group entries for single-texture groups and uses the generic createBindGroup(...) method.

Change-Id: I664957d4b724e54563a00178abe194453cdadf11
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1229036
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>