mesa: Check return value of __get_cpuid().

mesa: Check return value of __get_cpuid().

The use of the uninitialized_var() macro was to silence an uninitialized
variable warning that I assumed stemmed from gcc being unable to see
inside __get_cpuid() or understand its inline assembly.

In fact, it was because the __get_cpuid() function can fail, and not
initialize its arguments. Instead, check for failure and return early.

Reviewed-by: Brian Paul <brianp@vmware.com>