drm/amd/display/dce_mem_input: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce/dce_mem_input.c

@@ -479,7 +479,7 @@ static void program_grph_pixel_format(
 	case SURFACE_PIXEL_FORMAT_GRPH_ABGR16161616F:
 		sign = 1;
 		floating = 1;
-		/* no break */
+		/* fall through */
 	case SURFACE_PIXEL_FORMAT_GRPH_ARGB16161616F: /* shouldn't this get float too? */
 	case SURFACE_PIXEL_FORMAT_GRPH_ARGB16161616:
 		grph_depth = 3;