Revert "minigbm: Revert "minigbm: flush buffer instead of unmapping""

The reason the tests weren't passing is because DRM_IOCTL_I915_GEM_SET_DOMAIN
is required on x86 platforms when calling (*lock). A previous CLs added this.

BUG=b:67073097, b:67331142
TEST=

android.view.cts.SurfaceViewSyncTests
android.video.cts.VideoEncoderDecoderTest#testAvcGoog0Qual0720x0480
android.video.cts.VideoEncoderDecoderTest#testAvcGoog0Qual1280x0720
android.video.cts.VideoEncoderDecoderTest#testAvcGoog0Qual1920x1080
android.media.cts.EncodeDecodeTest#testVP8EncodeDecodeVideoFromSurfaceToSurface720p
android.media.cts.EncodeDecodeTest#testEncodeDecodeVideoFromPersistentSurfaceToSurface720p
android.media.cts.EncodeDecodeTest#testVP8EncodeDecodeVideoFromPersistentSurfaceToSurface720p

pass on Eve.

This reverts commit 14033e0013b5cc0ffc5af59dda7c377d46ec761d.

v2: Make lock_data[0] null after last (*unlock)(), because user may specify different
    access regions in the next (*lock)().

Change-Id: Ia56ecd57acc4471b026c852b178d8bc0f189dec7
Reviewed-on: https://chromium-review.googlesource.com/710324
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
diff --git a/gbm.c b/gbm.c
index c67bdcd..25b4fa0 100644
--- a/gbm.c
+++ b/gbm.c
@@ -249,7 +249,7 @@
 PUBLIC void gbm_bo_unmap(struct gbm_bo *bo, void *map_data)
 {
 	assert(bo);
-	drv_bo_unmap(bo->bo, map_data);
+	drv_bo_flush(bo->bo, map_data);
 }
 
 PUBLIC uint32_t gbm_bo_get_width(struct gbm_bo *bo)