drm_hwcomposer: Move platform-specific importer headers

Now that importer.h is platform.h, move the platform-specific
headers to match.

BUG=b/28117135
TEST=compiles

Change-Id: I08563505b77529257892841845ef4117d2992bb0
Signed-off-by: Sean Paul <seanpaul@chromium.org>
diff --git a/nvimporter.cpp b/nvimporter.cpp
index acf56ee..0ec455a 100644
--- a/nvimporter.cpp
+++ b/nvimporter.cpp
@@ -18,7 +18,7 @@
 
 #include "drmresources.h"
 #include "platform.h"
-#include "nvimporter.h"
+#include "platformnv.h"
 
 #include <cinttypes>
 #include <stdatomic.h>