drm/mxsfb: Add pm_runtime calls to pipe_enable/disable

Adding lcdif nodes to a power domain currently results in
black/corrupted screens or hangs because power is not correctly enabled
when required.

Ensure power is on when display is active by adding
pm_runtime_get/put_sync to mxsfb_pipe_enable/disable.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/ee88148399c63494cda4129b05444b0ac331b7a7.1537191359.git.leonard.crestez@nxp.com