drm/i915/cnl: Allow dynamic cdclk changes on CNL

All the low level cdclk bits are present, so let's add the required
hooks to reconfigure cdclk on the fly.

Cannonlake also needs to adjust the minimal pixel rate
as gen9 platforms. Specially for the Azalia audio case.

v2: Rebase due to cnl_sanitize_cdclk()
v3: Rebased by Rodrigo on top of Ville's cdclk rework.
v4: Rebase moving cnl_calc_cdclk up to follow same order
    as previous platforms.
v2: Squash drm/i915/cnl: Adjust min pixel rate. to address
    the current limitation where CDCLK cannot be set to 168MHz
    if audio is used with 96MHz. (Imre)
v3: adjust some of the clock limits within
    bdw_adjust_min_pipe_pixel_rate. (Ville/DK/Imre).
    Fix commit message messed by squash.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Sanyog Kale <sanyog.r.kale@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1497047175-27250-4-git-send-email-rodrigo.vivi@intel.com