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

drivers/gpu/drm/i915/display/i9xx_plane.c

@@ -1033,10 +1033,13 @@ i9xx_get_initial_plane_config(struct intel_crtc *crtc,
 					       DSPLINOFF(i9xx_plane));
 		base = intel_de_read(dev_priv, DSPSURF(i9xx_plane)) & DISP_ADDR_MASK;
 	} else {
+		offset = 0;
 		base = intel_de_read(dev_priv, DSPADDR(i9xx_plane));
 	}
 	plane_config->base = base;
 
+	drm_WARN_ON(&dev_priv->drm, offset != 0);
+
 	val = intel_de_read(dev_priv, PIPESRC(pipe));
 	fb->width = REG_FIELD_GET(PIPESRC_WIDTH_MASK, val) + 1;
 	fb->height = REG_FIELD_GET(PIPESRC_HEIGHT_MASK, val) + 1;

drivers/gpu/drm/i915/display/skl_universal_plane.c

@@ -2529,6 +2529,7 @@ skl_get_initial_plane_config(struct intel_crtc *crtc,
 	plane_config->base = base;
 
 	offset = intel_de_read(dev_priv, PLANE_OFFSET(pipe, plane_id));
+	drm_WARN_ON(&dev_priv->drm, offset != 0);
 
 	val = intel_de_read(dev_priv, PLANE_SIZE(pipe, plane_id));
 	fb->height = REG_FIELD_GET(PLANE_HEIGHT_MASK, val) + 1;