drm/mxsfb: Move axi clk enable/disable to crtc enable/disable

The main axi clk is disabled at the end of mxsfb_crtc_mode_set_nofb and
immediately reenabled in mxsfb_enable_controller.

Avoid this by moving the handling of axi clk one level up to
mxsfb_crtc_enable. Do the same for mxsfb_crtc_disable for symmetry.

This shouldn't have any functional effect.

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/985c1f1cad250bd9ca154b3e4b3f913c310eeabd.1537191359.git.leonard.crestez@nxp.com