drm/i915: Extract intel_crtc_scanline_offset()

Pull the scanline_offset calculation into its own function. Might
have further use for this later with DSB scanline waits.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230310235828.17439-4-ville.syrjala@linux.intel.com