drm/i915/rkl: Add DPLL4 support

drm/i915/rkl: Add DPLL4 support

Rocket Lake has a third DPLL (called 'DPLL4') that must be used to
enable a third display.  Unlike EHL's variant of DPLL4, the RKL variant
behaves the same as DPLL0/1.  And despite its name, the DPLL4 registers
are offset as if it were DPLL2.

v2:
 - Add new .update_ref_clks() hook.

v3:
 - Renumber TBT PLL to '3' and switch _MMIO_PLL3 to _MMIO_PLL (Lucas)

v4:
 - Don't drop _MMIO_PLL3; although it's now unused, we're going to need
   it very soon again for upcoming DG1 patches.  (Lucas)

v5:
 - Don't re-number TBT PLL and beyond, just use new RKL_DPLL_CFGCR
   macros to lookup the proper registers instead.  Although renumbering
   the PLLs might be something we want to consider down the road, it
   opens a big can of worms right now since a bunch of places in the
   code have an assumption that the PLL table has idx==id and no holes.
   Renumbering creates a hole for TGL, so we'd either need to allow
   holes in the table or break the idx==id invariant, both of which are
   somewhat invasive changes to the design.

Bspec: 49202
Bspec: 49443
Bspec: 50288
Bspec: 50289
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716220551.2730644-4-matthew.d.roper@intel.com
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>