drm/i915/plane: warn on non-zero plane offset

We assume the plane offset is 0. Warn if it's not. This also fixes a
warn on unused but set variable offset.

v2: initialize offset on the gen2/3 path (Ville)

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230526172218.1597394-1-jani.nikula@intel.com