HWC: Add a color transform attribute per display

Adds a color transform option to the queryable display attributes,
which allows a different vendor-specific color transform (e.g., wide
gamut, sRGB, etc.) per display configuration. This does not increase the
HWC version, and is considered a part of HWC 1.5, though if
getDisplayAttributes returns an error code as a result of being passed
an unsupported attribute, the call will be retried without the new color
transform option.

Bug: 20853317
Change-Id: Iab1605bc073ed4caa780e55ddf396efeeff3aff1

include/hardware/hwcomposer_defs.h

@@ -192,6 +192,10 @@ enum {
      */
     HWC_DISPLAY_DPI_X                       = 4,
     HWC_DISPLAY_DPI_Y                       = 5,
+
+    /* Indicates which of the vendor-defined color transforms is provided by
+     * this configuration. */
+    HWC_DISPLAY_COLOR_TRANSFORM             = 6,
 };
 
 /* Allowed events for hwc_methods::eventControl() */