drm_hwcomposer: Use mode vrefresh if provided

Instead of always calculating the vrefresh from clock + resolution,
use vrefresh from the mode if its provided.

Bug: 25319209
Test: Tested on smaug, refresh reports 60Hz

Change-Id: I963ccb153d1856f5e45e4a1aa6fbd20b105d4a84
Signed-off-by: Sean Paul <seanpaul@chromium.org>
diff --git a/drmmode.h b/drmmode.h
index 3088b7a..1110ce8 100644
--- a/drmmode.h
+++ b/drmmode.h
@@ -71,6 +71,7 @@
   uint32_t v_sync_end_;
   uint32_t v_total_;
   uint32_t v_scan_;
+  uint32_t v_refresh_;
 
   uint32_t flags_;
   uint32_t type_;