drm/plane-helper: Add drm_plane_helper_check_update() (v3)

Pull the parameter checking from drm_primary_helper_update() out into
its own function; drivers that provide their own setplane()
implementations rather than using the helper may still want to share
this parameter checking logic.

A few of the checks here were also updated based on suggestions by
Ville Syrjälä.

v3:
 - s/primary_helper/plane_helper/ --- this checking logic may be useful
   for other types of planes as well.
 - Fix visibility check (need to dereference visibility pointer)
v2:
 - Pass src/dest/clip rects and min/max scaling down to helper to avoid
   duplication of effort between helper and drivers (suggested by
   Ville).
 - Allow caller to specify whether the primary plane should be
   updatable while the crtc is disabled.

Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Chon Ming Lee <chon.ming.lee@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
[danvet: Include header properly and fixup declaration mismatch to
make this compile.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_plane_helper.h

@@ -24,6 +24,17 @@
 #ifndef DRM_PLANE_HELPER_H
 #define DRM_PLANE_HELPER_H
 
+#include <drm/drm_rect.h>
+
+/*
+ * Drivers that don't allow primary plane scaling may pass this macro in place
+ * of the min/max scale parameters of the update checker function.
+ *
+ * Due to src being in 16.16 fixed point and dest being in integer pixels,
+ * 1<<16 represents no scaling.
+ */
+#define DRM_PLANE_HELPER_NO_SCALING (1<<16)
+
 /**
  * DOC: plane helpers
  *
@@ -31,6 +42,17 @@
  * planes.
  */
 
+extern int drm_plane_helper_check_update(struct drm_plane *plane,
+					 struct drm_crtc *crtc,
+					 struct drm_framebuffer *fb,
+					 struct drm_rect *src,
+					 struct drm_rect *dest,
+					 const struct drm_rect *clip,
+					 int min_scale,
+					 int max_scale,
+					 bool can_position,
+					 bool can_update_disabled,
+					 bool *visible);
 extern int drm_primary_helper_update(struct drm_plane *plane,
 				     struct drm_crtc *crtc,
 				     struct drm_framebuffer *fb,