Add automatic generation of mipmaps to Vulkan

BUG=skia:
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Review URL: https://codereview.chromium.org/1916563002
diff --git a/src/gpu/vk/GrVkCaps.cpp b/src/gpu/vk/GrVkCaps.cpp
index 5b62a37..b0f3acf 100644
--- a/src/gpu/vk/GrVkCaps.cpp
+++ b/src/gpu/vk/GrVkCaps.cpp
@@ -20,7 +20,7 @@
     /**************************************************************************
     * GrDrawTargetCaps fields
     **************************************************************************/
-    fMipMapSupport = false; //TODO: figure this out
+    fMipMapSupport = true;   // always available in Vulkan
     fNPOTTextureTileSupport = false; //TODO: figure this out
     fTwoSidedStencilSupport = false; //TODO: figure this out
     fStencilWrapOpsSupport = false; //TODO: figure this out