drm/i915: Move SKL+ DBUF enable/disable to display core init/uninit

drm/i915: Move SKL+ DBUF enable/disable to display core init/uninit

SKL and BXT have the same snippets of code for enabling disabling the
DBUF. Extract those into helpers and move the calls from
init/unit_cdclk() to the display core init/init since this stuff isn't
really about cdclk. Also doing the enable twice shouldn't hurt since
you're just setting the request bit again when it was already set.

We can also toss in a few WARNs about the register values into
skl_get_dpll0_vco() now that we know that things should always be
sane there.

Flatten skl_init_cdclk() while at it.

v2: s/skl/gen9/ in function names (Imre)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-12-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>