video: mxsfb: remove dotclk_delay from platform_data

There is no in-tree mxsfb users using mxsfb_platform_data dotclk_delay.
Let's remove it from mxsfb_platform_data to ease full device tree
adoption of mxsfb driver.  If later we have platform/board need to
configure this parameter, we can add it into device tree bindings.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>