drm: fix spelling mistake: "committing"

Trivial fix to spelling mistake in DRM_DEBUG_ATOMIC debug message

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170412162722.25087-1-colin.king@canonical.com