drm/i915: Reduce CHV DPLL min vco frequency to 4.8 GHz

drm/i915: Reduce CHV DPLL min vco frequency to 4.8 GHz

The current minimum vco frequency leaves us with a gap in our supported
frequencies at 233-243 MHz. Your typical 2560x1440@60 display wants a
pixel clock of 241.5 MHz, which is just withing that gap. Reduce the
allowed vco min frequency to 4.8GHz to reduce the gap to 233-240 MHz,
and thus allow such displays to work.

4.8 GHz is actually the documented (at least in some docs) limit of the
PLL, and we just picked 4.86 GHz originally because that was the lowest
value produced by the PLL spreadsheet, which obviously didn't consider
2560x1440 displays.

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