sna: Add some missing 'fall through' comments

Static analysers are dumb and presume readers are too.

Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/gen3_render.c

@@ -1065,6 +1065,7 @@ gen3_composite_emit_shader(struct sna *sna,
 		case SHADER_LINEAR:
 		case SHADER_RADIAL:
 			gen3_fs_dcl(FS_S0 + t);
+			/* fall through */
 		case SHADER_OPACITY:
 			gen3_fs_dcl(FS_T0 + t);
 			break;

src/sna/sna_dri.c

@@ -1075,6 +1075,7 @@ static void sna_dri_vblank_handle(int fd,
 						 get_private(info->back)->bo,
 						 true);
 		info->type = DRI2_SWAP_THROTTLE;
+		/* fall through to SwapComplete */
 	case DRI2_SWAP_THROTTLE:
 		DBG(("%s: %d complete, frame=%d tv=%d.%06d\n",
 		     __FUNCTION__, info->type, frame, tv_sec, tv_usec));

src/sna/sna_render.h

@@ -589,6 +589,7 @@ sna_get_pixel_from_rgba(uint32_t * pixel,
 	switch (format) {
 	case PICT_x8r8g8b8:
 		alpha = 0xffff;
+		/* fall through to re-use a8r8g8b8 expansion */
 	case PICT_a8r8g8b8:
 		*pixel = ((alpha >> 8 << 24) |
 			  (red >> 8 << 16) |