drm_hwcomposer: Allow pre-multi layer to always pass DrmPlane validation

This logic was removed accidentaly during cleanup. Restore it.
Enables hardware-accelerated composition on raspberry-pi 4.

Fixes: 5063d53ddf7d ("drm_hwcomposer: Improve handling of blending mode")
Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
diff --git a/drm/DrmPlane.cpp b/drm/DrmPlane.cpp
index c75a5af..d39b0cc 100644
--- a/drm/DrmPlane.cpp
+++ b/drm/DrmPlane.cpp
@@ -224,7 +224,8 @@
   }
 
   if (blending_enum_map_.count(layer->blending) == 0 &&
-      layer->blending != DrmHwcBlending::kNone) {
+      layer->blending != DrmHwcBlending::kNone &&
+      layer->blending != DrmHwcBlending::kPreMult) {
     ALOGV("Blending is not supported on plane %d", id_);
     return false;
   }