drm/i915: Skip modeset for cdclk changes if possible

If we have only a single active pipe and the cdclk change only requires
the cd2x divider to be updated bxt+ can do the update with forcing a full
modeset on the pipe. Try to hook that up.

v2:
- Wait for vblank after an optimized CDCLK change.
- Avoid optimization if the pipe needs a modeset (or was disabled).
- Split CDCLK change to a pre/post plane update step.
v3:
- Use correct version of CDCLK state as old state. (Ville)
- Remove unused intel_cdclk_can_skip_modeset()
v4:
- For consistency call intel_set_cdclk_post_plane_update() only during
  modesets (and not fastsets).
v5:
- Remove the logic to update the CD2X divider on-the-fly on ICL, since
  only a divider of 1 is supported there. Clint also noticed that the
  pipe select bits in CDCLK_CTL are oddly defined on ICL, it's not clear
  yet whether that's only an error in the specification.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Abhay Kumar <abhay.kumar@intel.com>
Tested-by: Abhay Kumar <abhay.kumar@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190327101321.3095-1-imre.deak@intel.com