drm_hwcomposer: Convert v_refresh() to float

We convert v_refresh() to return a float and instead of storing it,
compute it on the fly from the pixel clock + htotal/vtotal. This makes
the synthetic vblank computation much more accurate.

Change-Id: I2a0becf75eaca8ace30d176fdc813f3b57ba23a5
diff --git a/drmmode.h b/drmmode.h
index ab213ef..3088b7a 100644
--- a/drmmode.h
+++ b/drmmode.h
@@ -48,7 +48,7 @@
   uint32_t v_sync_end() const;
   uint32_t v_total() const;
   uint32_t v_scan() const;
-  uint32_t v_refresh() const;
+  float v_refresh() const;
 
   uint32_t flags() const;
   uint32_t type() const;
@@ -71,7 +71,6 @@
   uint32_t v_sync_end_;
   uint32_t v_total_;
   uint32_t v_scan_;
-  uint32_t v_refresh_;
 
   uint32_t flags_;
   uint32_t type_;