drm_hwcomposer: Fix alpha check

This change fixes a check which determines whether a layer's alpha
is set to opaque. The PresentInfo::alpha property was recently
converted to float, but the check in DrmPlane::IsValidForLayer was
still comparing it to the integer value.

Change-Id: Ie1a8304d7302d1d646d0542be92795e93f57e0d9
Signed-off-by: Andrew Wolfers <aswolfers@google.com>

drm/DrmPlane.cpp

diff --git a/drm/DrmPlane.cpp b/drm/DrmPlane.cpp
index 36f29b7..8f31ee5 100644
--- a/drm/DrmPlane.cpp
+++ b/drm/DrmPlane.cpp
@@ -189,7 +189,7 @@
     return false;
   }
 
-  if (!alpha_property_ && layer->pi.alpha != UINT16_MAX) {
+  if (!alpha_property_ && layer->pi.alpha != kAlphaOpaque) {
     ALOGV("Alpha is not supported on plane %d", GetId());
     return false;
   }