drm_hwcomposer: Make sure we check for new 16bit alpha value

In commit 025d0a65 ("Use 16 bit value for plane alpha"),
the alpha value was bumped to a 16bit value.

Unfortunately in drmdisplaycompositor.cpp we are still
comparing it against a 8bit 0xff value, which now fails
causing the display to not start.

So fix this comparing to the 16bit 0xffff value.

With this change the display starts up again.

Change-Id: Ic796b16fb57ae7cea58b5c66c456a58e3fb57ba5
Signed-off-by: John Stultz <john.stultz@linaro.org>
diff --git a/drmdisplaycompositor.cpp b/drmdisplaycompositor.cpp
index 0b4e2b2..d2acf76 100644
--- a/drmdisplaycompositor.cpp
+++ b/drmdisplaycompositor.cpp
@@ -626,7 +626,7 @@
     }
 
     // TODO: Once we have atomic test, this should fall back to GL
-    if (alpha != 0xFF && plane->alpha_property().id() == 0) {
+    if (alpha != 0xFFFF && plane->alpha_property().id() == 0) {
       ALOGE("Alpha is not supported on plane %d", plane->id());
       ret = -EINVAL;
       break;