drm/i915: Introduce intel_crtc_scanline_to_hw()

Add a helper to convert our idea of a scanline to the hw's idea
of the same scanline (ie. apply crtc->scanline_offset in reverse).
We'll need this to tell the DSB do stuff on a specific scanline.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230606191504.18099-17-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>