drm/i915: Keep the global DPLL state in a DPLL specific struct

For clarity add a new DPLL specific struct to the i915 device struct and
move all DPLL fields into it. Accordingly remove the dpll_ prefixes, as
the new struct already provides the required namespacing.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226203455.23032-4-imre.deak@intel.com