i915: Use 120MHz LVDS SSC clock for gen5/gen6/gen7

We had been using a DMI table workaround to select the right
frequency for devices, but this is fragile and must be updated
with every new platform.

Instead the default case when VBT is missing is changed to use
120MHz clock for LVDS SSC for these generations.

The docs for 2010-Core, SandyBridge, and IvyBridge all indicate
that the reference frequency for LVDS is 120MHz:

"2010 Core"
http://intellinuxgraphics.org/IHD_OS_Vol3_Part3r2.pdf
page 38
Reference Frequency: 120MHz for CRT and LVDS.  100MHz for the FDI.

"2011 SandyBridge"
http://intellinuxgraphics.org/documentation/SNB/IHD_OS_Vol3_Part3.pdf
page 33
Reference Frequency: 120MHz for CRT, HDMI, LVDS.  100MHz for the FDI.

"2012 IvyBridge"
http://intellinuxgraphics.org/documentation/IVB/IHD_OS_Vol3_Part4.pdf


page 27
Reference Frequency: 120 MHz for CRT, HDMI, LVDS, 100MHz for the FDI.

Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
[olof: Fixup for recent base, switched from if/else to single call]
Signed-off-by: Olof Johansson <olof@lixom.net>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>