drm/i915: Extract intel_crtc_compute_pipe_mode()

Pull intel_crtc_compute_pipe_mode() out from
intel_crtc_compute_config(). Since it's semi related
we'll suck in the max dotclock/double wide checks in
as well.

And we'll pimp the debugs while at it.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223131315.18016-9-ville.syrjala@linux.intel.com