drm/i915/dpll: add intel_dpll_crtc_compute_clock()

Avoid referencing the function pointer directly to be able to abstract
the call better.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/12ac40c1ef9915fe680c6657b603b08be022626b.1643896905.git.jani.nikula@intel.com