display: fix get_cpuid_max clobbering %rbx (fixes Py 3.8 crash on macOS)

This could cause a crash when constructing a GraphicsPipe() under some conditions (observed in Python 3.8).  Credit goes to @CFSworks for tracking this down.

panda/src/display/graphicsPipe.cxx

@@ -55,25 +55,6 @@ union cpuid_info {
   };
 };
 
-/**
- * Returns the highest cpuid leaf that is supported by the CPU.
- */
-static inline uint32_t get_cpuid_max(uint32_t leaf) {
-#if defined(__GNUC__) && !defined(__APPLE__)
-  return __get_cpuid_max(leaf, nullptr);
-#elif defined(_MSC_VER)
-  uint32_t p[4] = {0};
-  __cpuid((int *)p, leaf);
-  return p[0];
-#else
-  unsigned int eax = 0;
-  __asm__ ("cpuid\n\t"
-           : "=a" (eax)
-           : "0" (leaf));
-  return eax;
-#endif
-}
-
 /**
  * Gets cpuid info for the given leaf.
  */
@@ -88,6 +69,19 @@ static inline void get_cpuid(uint32_t leaf, cpuid_info &info) {
            : "0" (leaf));
 #endif
 }
+
+/**
+ * Returns the highest cpuid leaf that is supported by the CPU.
+ */
+static inline uint32_t get_cpuid_max(uint32_t leaf) {
+#if defined(__GNUC__) && !defined(__APPLE__)
+  return __get_cpuid_max(leaf, nullptr);
+#else
+  cpuid_info info;
+  get_cpuid(leaf, info);
+  return info.eax;
+#endif
+}
 #endif
 
 #ifdef IS_LINUX