drm/i915: Make {vlv,chv}_{disable,update}_pll() more similar

drm/i915: Make {vlv,chv}_{disable,update}_pll() more similar

The VLV and CHV DPLL disable and update are almost identical in
how the DPLL/DPLL_MD registers need to be set up. But the code
looks more different than it really is. Try to bring them into
line.

Note that we now leave the refclock always enabled for both
DPLLs in the dual channel PHY. But that's perfectly fine since
it's the same clock, and we anyway already do that when turning
the disp2d power well on.

v2: s/chv_update_pll/chv_compute_dpll/
v3: Add a note that we leave refclocks enabled for both DPLLs (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458052809-23426-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>