drm/i915: Enable DisplayPort in Valleyview

In valleyview voltageswing, pre-emphasis and lane control registers can
be programmed only through the h/w side band fabric.

Cleaned up DPLL calculations for Valleyview to support multi display
configurations.

v2: Based on Daniel's feedbacak, moved crt hotplug detect work around as separate
patch. Also moved i9xx_update_pll_dividers to i8xx_update_pll and
i9xx_update_pll.

Signed-off-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Signed-off-by: Gajanan Bhat <gajanan.bhat@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: drop spurious whitespace changes.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>