drm/msm/gem: Convert to drm_exec

Replace the ww_mutex locking dance with the drm_exec helper.

v2: Error path fixes, move drm_exec_fini so we only call it once (and
    only if we have drm_exec_init()

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/568342/