drm/i915/skl: Define shared DPLLs for Skylake

drm/i915/skl: Define shared DPLLs for Skylake

On skylake, DPLL 1, 2 and 3 can be used for DP and HDMI. The shared dpll
framework allows us to share those DPLLs among DDIs when possible.

The most tricky part is to provide a DPLL state that can be easily
compared. DPLL_CRTL1 is shared by all the DPLLs, 6 bits each. The
per-dpll crtl1 field of the hw state is then normalized to be the same
value if 2 DPLLs do indeed have identical values for those 6 bits.

v2: Port the code to the shared DPLL infrastructure (Damien)

v3: Rebase on top of Ander's clock computation staging work for atomic (Damien)

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v2)
Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com> (v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>