video: mxsfb: refactor for using display_timings

video: mxsfb: refactor for using display_timings

struct display_timings provides more informations such clock and DE
polarity, so let's refactor the code to use struct display_timings
instead of struct ctfb_res_modes, so we'll become able to get clock and
DE polarity settings and set register according to them in the next patch.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>