drm/i915: Disable CLKOUT_DP bending on LPT/WPT as needed

drm/i915: Disable CLKOUT_DP bending on LPT/WPT as needed

When we want to use SPLL for FDI we want SSC, which means we have to
disable clock bending for the PCH SSC reference (bend and spread are
mutually exclusive). So let's turn off bending when we want spread.
In case the BIOS enabled clock bending for some reason we'll just turn
it off and enable the spread mode instead.

Not sure what happens if the BIOS is actually using the bend source for
HDMI at this time, but I suppose it should be no worse than what already
happens when we simply turn on the spread.

We don't currently use the bend source for anything, and only use the
PCH SSC reference for the SPLL to drive FDI (always with spread).

v2: Fix the %5 vs %10 fumble for SSCDITHPHASE (Paulo)
    Add 'WARN_ON(steps % 5 != 0)' sanity check (Paulo)
    Fix typos in commit message (Paulo)

Cc: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449260379-14093-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>