drm/i915: Add crtc .crtc_get_shared_dpll()

drm/i915: Add crtc .crtc_get_shared_dpll()

Start splitting the .compute_crtc_clock() into two parts; one
part does the computation, the second part does the shared dpll
assignment. I want to move the actual computation part much earlier
into the compute_config() phase.

v2: dg2_crtc_get_shared_dpll() not needed (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220325123205.22140-10-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>