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

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>