[media] video: mx2_camera: Use clk_prepare_enable/clk_disable_unprepare

Prepare the clock before enabling it.

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: <linux-media@vger.kernel.org>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>