minigbm/msm: Don't use uninitialized metadata to setup DRM_FORMAT_R8

minigbm/msm: Don't use uninitialized metadata to setup DRM_FORMAT_R8

CL:2245540 seems to have accidentally modified msm.c to use an
uninitialized metadata when setting up DRM_FORMAT_R8.  Let's go back
to using LINEAR_METADATA.

BUG=b:161426616
TEST=Camera no longer crashes

Change-Id: Ia587821216c82d4488b096686904f82147224aa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/2310289
Tested-by: Douglas Anderson <dianders@chromium.org>
Auto-Submit: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: David Stevens <stevensd@chromium.org>
Commit-Queue: Douglas Anderson <dianders@chromium.org>