imx: clock: gate clk before changing pix clk mux

The LCDIF Pixel clock mux is not glitchless, so need
to gate before changing mux.

Also change enable_lcdif_clock prototype with a new input
parameter to indicate disable or enable.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>