drm/i915/ehl: Add dpll mgr

Elkhart Lake has a different set of PLLs as compared to Ice Lake,
although programming them is very similar.

v2: Rebase on top of s/icl_pll_funcs/combo_pll_funcs

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322175847.25707-3-rodrigo.vivi@intel.com