Add kRGB_888X_GrPixelConfig

This is needed to support importing of RGBX AHB into skia's vulkan backend.
With this CL we only enable the new pixel config to be textureable.

Bug: skia:
Change-Id: Iba9180c14f3ef633ae846091cf453d68f82ce544
Reviewed-on: https://skia-review.googlesource.com/c/192035
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/GrContextPriv.cpp b/src/gpu/GrContextPriv.cpp
index b502bf8..1ba47c8 100644
--- a/src/gpu/GrContextPriv.cpp
+++ b/src/gpu/GrContextPriv.cpp
@@ -282,6 +282,7 @@
         case kRGBA_4444_GrPixelConfig:          return true;
         case kRGBA_8888_GrPixelConfig:          return true;
         case kRGB_888_GrPixelConfig:            return false;
+        case kRGB_888X_GrPixelConfig:           return false;
         case kRG_88_GrPixelConfig:              return false;
         case kBGRA_8888_GrPixelConfig:          return true;
         case kSRGBA_8888_GrPixelConfig:         return true;