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

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.

BUG=chrome-os-partner:11744
TEST=manual: boot on Lumpy and Parrot with and without VBIOS

Change-Id: Ieae3d37d4a5f6eb11c45a5e2fa76b0c106404e69
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/28274
Reviewed-by: Marc Jones <marc.jones@se-eng.com>