drm/i915: Make SKL/KBL DPLL0 managed by the shared dpll code

Include DPLL0 in the managed dplls for SKL/KBL. While it has to be kept
enabled because of it driving CDCLK, it is better to special case that
inside the DPLL code than in the higher level.

v2: Use INTEL_DPLL_ALWAYS_ON flag. (Ander)

v3: Remove extremely paranoid WARN_ONs. (Maarten)
    Handle DPLL0 in skylake_get_ddi_pll() properly. (Ander)

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457451987-17466-14-git-send-email-ander.conselvan.de.oliveira@intel.com