drm/i915: Add support for Kabylake to function obtaining shared PLL

This adds support for KBL in the new function added in commit ID:
commit <f169660e> that returns a
shared pll in case of DDI platforms.

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473728663-14355-1-git-send-email-manasi.d.navare@intel.com