drm/dp: annotate implicit fall throughs

There is a plan to build the kernel with -Wimplicit-fallthrough and
these places in the code produced warnings (W=1). Fix them up.

This commit remove the following warnings:

include/linux/compiler.h:77:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
include/asm-generic/bug.h:134:2: note: in expansion of macro 'unlikely'
drivers/gpu/drm/drm_dp_helper.c:155:3: note: in expansion of macro 'WARN'
include/linux/compiler.h:77:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
include/asm-generic/bug.h:134:2: note: in expansion of macro 'unlikely'
drivers/gpu/drm/drm_dp_helper.c:173:3: note: in expansion of macro 'WARN'
drivers/gpu/drm/drm_dp_helper.c:547:3: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Mathieu Malaterre <[e-mail]>
Acked-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_dp_helper.c

···
 	default:
 		WARN(1, "unknown DP link rate %d, using %x\n", link_rate,
 		     DP_LINK_BW_1_62);
+		/* fall through */
 	case 162000:
 		return DP_LINK_BW_1_62;
 	case 270000:
···
 	switch (link_bw) {
 	default:
 		WARN(1, "unknown DP link BW code %x, using 162000\n", link_bw);
+		/* fall through */
 	case DP_LINK_BW_1_62:
 		return 162000;
 	case DP_LINK_BW_2_7:
···
 		case DP_DS_16BPC:
 			return 16;
 		}
+		/* fall through */
 	default:
 		return 0;
 	}