drm/i915: Split shared dpll .get_dplls() into compute and get phases

Split the DPLL state computation into a separate function
from the current .get_dplls() which currently serves a dual duty
by also reserving the shared DPLLs.

v2: s/false/-EINVAL/ (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220503182242.18797-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>