i965: Use {} to initialize GENX_* structs.

i965: Use {} to initialize GENX_* structs.

gen4 have commands which start with KernelStartPointer, which is a
struct, so if we initialize it struct = { 0 }, we get warnings on some
compilers:

"GCC (pre 4.9?) can throw a Wmissing-braces on[1] while clang
-Wmissing-field-initializers [2]." - Emil

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119
[2] https://bugs.llvm.org/show_bug.cgi?id=21689

This change works around that and will silence such warnings. It is both
a GCC and a clang extension.

v2:
   - Use {} instead of memset macro (Matt)

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>