video: fb: xylonfb: Refresh rate bug fix

Fixed bug in calculation of display refresh rate when working in no EDID mode.
Added usage of VESA timings with reduced blanking period when getting video
mode timing parameters from kernel.

Signed-off-by: Davor Joja <davor.joja@logicbricks.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>