Set emulator framebuffer display device buffer count to 3

Otherwise, we get deadlock.

bug: 36439031

Change-Id: Icffeca757120deb2b1392d8fc3531e3737d7cdc4
diff --git a/target/board/generic/BoardConfig.mk b/target/board/generic/BoardConfig.mk
index 9cbe215..946e480 100644
--- a/target/board/generic/BoardConfig.mk
+++ b/target/board/generic/BoardConfig.mk
@@ -39,6 +39,7 @@
 endif
 
 TARGET_USES_HWC2 := true
+NUM_FRAMEBUFFER_SURFACE_BUFFERS := 3
 
 # Build OpenGLES emulation guest and host libraries
 BUILD_EMULATOR_OPENGL := true