drm/i915: Configure DPLL's for Cannonlake

DPLL's are defined in DPCLKA_CFGCR0 register (0x6C200). Let's use these
definitions when computing dpll's for ddi ports.

v2: (Rodrigo) Remove register that was defined in another patch with
    fixed name and more bits.

Signed-off-by: Kahola, Mika <mika.kahola@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1497047175-27250-6-git-send-email-rodrigo.vivi@intel.com