hw/xfree86: Calculate the clock and refresh rate based on panel's preferred mode

xserver fails to generate useable resolutions with 90Hz framerate panels(encounter the same issue with 3 different 2.5k resolution panels). All the resolutions shown by xrandr lead to blank screen except the one written in EDID. Ville Syrjälä from Intel provides a method to calculate the preferred clock and refresh rate from the existing resolution table and this works for the issue.

Closes: #1388 (closed) Signed-off-by: Chia-Lin Kao (AceLan) acelan.kao@canonical.com