drm/i915: Consider SPLL as another shared pll, v2.

When diagnosing a unrelated bug for someone on irc, it would seem the hardware can
be brought up by the BIOS with the embedded displayport using the SPLL for spread spectrum.

Right now this is not handled well in i915, and it calculates the crtc needs to
be reprogrammed on the first modeset without SSC, but  the SPLL itself was kept
active. Fix this by exposing SPLL as a shared pll that will not be returned
by intel_get_shared_dpll; you have to know it exists to use it.

Changes since v1:
- Create a separate dpll_hw_state.spll for spll, and use
  separate pll functions for spll.

Tested-by: Emil Renner Berthing <kernel@esmil.dk>
Tested-by: Gabriel Feceoru <gabriel.feceoru@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447681332-6318-1-git-send-email-maarten.lankhorst@linux.intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>