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

src/dsp/cost.c

@@ -395,12 +395,12 @@ WEBP_DSP_INIT_FUNC(VP8EncDspCostInit) {
       VP8EncDspCostInitMIPSdspR2();
     }
 #endif
-#if defined(WEBP_USE_SSE2)
+#if defined(WEBP_HAVE_SSE2)
     if (VP8GetCPUInfo(kSSE2)) {
       VP8EncDspCostInitSSE2();
     }
 #endif
-#if defined(WEBP_USE_NEON)
+#if defined(WEBP_HAVE_NEON)
     if (VP8GetCPUInfo(kNEON)) {
       VP8EncDspCostInitNEON();
     }