dsp/*: use WEBP_HAVE_* to determine Init availability

after:
  ece18e55 dsp.h: respect --disable-sse2/sse4.1/neon
WEBP_USE_* will be set when a module is targeting a particular
instruction set, e.g., sse4.1, and not overridden if WEBP_HAVE_SSE41 is
set, as previously this would ignore the case where the instruction set
was disabled via config.h and the HAVE macro was unset.

dsp.h not ensures WEBP_HAVE_* are set when WEBP_USE_* to cover cases
where the files are built without config.h.

Change-Id: Ia1c2dcf4100cc1081d968acb6e085e2a1768ece6
(cherry picked from commit 1fe3162541)

src/dsp/ssim.c

@@ -150,7 +150,7 @@ WEBP_DSP_INIT_FUNC(VP8SSIMDspInit) {
 #endif
 
   if (VP8GetCPUInfo != NULL) {
-#if defined(WEBP_USE_SSE2)
+#if defined(WEBP_HAVE_SSE2)
     if (VP8GetCPUInfo(kSSE2)) {
       VP8SSIMDspInitSSE2();
     }