drm: sti: sti_hqvdp: undo preparation of a clock source.

Undo preparation of a clock source, if sti_hqvdp_start_xp70 and
sti_hqvdp_atomic_check are not successful.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/7afad3012fb6e40f43a1eb5a64dc6364c38bd052.1498039961.git.arvind.yadav.cs@gmail.com