drm/i915: Split CHV and VLV specific crtc_compute_clock() hooks

In order for VLV and CHV to use i9xx_crtc_compute_clocks(), a number
of if ladders is necessary: one for setting the find_dpll() hook, one
for choosing the limits struct, one for choosing the right compute dpll
function and one for initializing the crtc_compute_clock() hook.

By extracting a platform specific implementation for each platform, the
number of if-ladders is reduced to one.

While at it also clean up bxt_find_best_dpll() which depends on some of
the CHV code.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458576016-30348-13-git-send-email-ander.conselvan.de.oliveira@intel.com