drm/i915: Use drm_rect_translate_to()

Use the newly introduced drm_rect_translate_to() instead
of hand rolling it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190930134214.24702-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>