drm/i915: Allow pixel clock up to 95% of cdclk on CHV

Supposedly CHV can sustain a pixel clock of up to 95% of
cdclk, as opposed to the 90% limit that was used old older
platforms. Update the cdclk selection code to allow for this.

This will allow eg. HDMI 4k modes with their 297MHz pixel clock
while still respecting the 320 MHz cdclk limit on CHV.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Vijay Purushothaman <vijay.a.purushothaman@linux.intel.com>
Reviewed-by: Yogesh Mohan Marimuthu <yogesh.mohan.marimuthu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>