drm/ast: Add drm_panic support

Add support for the drm_panic module, which displays a message to
the screen when a kernel panic occurs.

v7
* Use drm_for_each_primary_visible_plane()

v8:
* Replace get_scanout_buffer() logic with drm_panic_set_buffer()
(Thomas Zimmermann)
v9:
* Revert to using get_scanout_buffer() (Sima)
* move get_scanout_buffer() to plane helper functions

v12:
* Use array for map and pitch in struct drm_scanout_buffer
to support multi-planar format later. (Thomas Zimmermann)

Signed-off-by: Jocelyn Falempe <[e-mail]>
Acked-by: Sui Jingfeng <[e-mail]>
Tested-by: Sui Jingfeng <[e-mail]>
Reviewed-by: Thomas Zimmermann <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Acked-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/ast/ast_mode.c

···
 #include <drm/drm_gem_framebuffer_helper.h>
 #include <drm/drm_gem_shmem_helper.h>
 #include <drm/drm_managed.h>
+#include <drm/drm_panic.h>
 #include <drm/drm_probe_helper.h>
 #include <drm/drm_simple_kms_helper.h>
 
···
 	ast_set_index_reg_mask(ast, AST_IO_VGASRI, 0x1, 0xdf, 0x20);
 }
 
+static int ast_primary_plane_helper_get_scanout_buffer(struct drm_plane *plane,
+						       struct drm_scanout_buffer *sb)
+{
+	struct ast_plane *ast_plane = to_ast_plane(plane);
+
+	if (plane->state && plane->state->fb && ast_plane->vaddr) {
+		sb->format = plane->state->fb->format;
+		sb->width = plane->state->fb->width;
+		sb->height = plane->state->fb->height;
+		sb->pitch[0] = plane->state->fb->pitches[0];
+		iosys_map_set_vaddr_iomem(&sb->map[0], ast_plane->vaddr);
+		return 0;
+	}
+	return -ENODEV;
+}
+
 static const struct drm_plane_helper_funcs ast_primary_plane_helper_funcs = {
 	DRM_GEM_SHADOW_PLANE_HELPER_FUNCS,
 	.atomic_check = ast_primary_plane_helper_atomic_check,
 	.atomic_update = ast_primary_plane_helper_atomic_update,
 	.atomic_enable = ast_primary_plane_helper_atomic_enable,
 	.atomic_disable = ast_primary_plane_helper_atomic_disable,
+	.get_scanout_buffer = ast_primary_plane_helper_get_scanout_buffer,
 };
 
 static const struct drm_plane_funcs ast_primary_plane_funcs = {