d3d12: Fix implicit fallthrough warnings

Changes comments to use FALLTHROUGH macro, to silence warnings
from both GCC and clang.
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <mesa/mesa!8248>

src/gallium/drivers/d3d12/d3d12_compiler.cpp

@@ -915,7 +915,7 @@ get_prev_shader(struct d3d12_context *ctx, pipe_shader_type current)
    case PIPE_SHADER_FRAGMENT:
       if (ctx->gfx_stages[PIPE_SHADER_GEOMETRY])
          return ctx->gfx_stages[PIPE_SHADER_GEOMETRY];
-      /* fallthrough */
+      FALLTHROUGH;
    case PIPE_SHADER_GEOMETRY:
       return ctx->gfx_stages[PIPE_SHADER_VERTEX];
    default:
@@ -932,7 +932,7 @@ get_next_shader(struct d3d12_context *ctx, pipe_shader_type current)
    case PIPE_SHADER_VERTEX:
       if (ctx->gfx_stages[PIPE_SHADER_GEOMETRY])
          return ctx->gfx_stages[PIPE_SHADER_GEOMETRY];
-      /* fallthrough */
+      FALLTHROUGH;
    case PIPE_SHADER_GEOMETRY:
       return ctx->gfx_stages[PIPE_SHADER_FRAGMENT];
    case PIPE_SHADER_FRAGMENT:
@@ -963,7 +963,7 @@ scan_texture_use(nir_shader *nir)
                case nir_texop_txd:
                   if (tex->is_shadow)
                      result |= TEX_CMP_WITH_LOD_BIAS_GRAD;
-                  /* fallthrough */
+                  FALLTHROUGH;
                case nir_texop_tex:
                   if (tex->dest_type & (nir_type_int | nir_type_uint))
                      result |= TEX_SAMPLE_INTEGER_TEXTURE;

src/gallium/drivers/d3d12/d3d12_format.c

@@ -26,6 +26,7 @@
 #include "pipe/p_format.h"
 #include "util/format/u_format.h"
 #include "util/u_math.h"
+#include "util/compiler.h"
 
 static const DXGI_FORMAT formats[PIPE_FORMAT_COUNT] = {
 #define MAP_FORMAT_NORM(FMT) \
@@ -180,7 +181,7 @@ d3d12_get_resource_srv_format(enum pipe_format f, enum pipe_texture_target targe
    case PIPE_FORMAT_A8_UNORM:
       if (target == PIPE_BUFFER)
          return DXGI_FORMAT_R8_UNORM; /* A8_UNORM is not supported for buffer SRV */
-      /* passthrough */
+      FALLTHROUGH;
    default:
       return d3d12_get_format(f);
    }

src/gallium/drivers/d3d12/d3d12_pipeline_state.cpp

@@ -64,7 +64,7 @@ get_semantic_name(int slot, unsigned *index)
 
    case VARYING_SLOT_CLIP_DIST1:
       *index = 1;
-      /* fallthrough */
+      FALLTHROUGH;
    case VARYING_SLOT_CLIP_DIST0:
       return "SV_ClipDistance";