drm/i915: Extract intel_crtc_compute_pipe_src()

intel_crtc_compute_config() doesn't really tell a unified story.
Let's chunk it up into pieces. We'll start with
intel_crtc_compute_pipe_src().

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/20220223131315.18016-8-ville.syrjala@linux.intel.com