anv/gem: Use C99-style struct initializers for DRM structs

anv/gem: Use C99-style struct initializers for DRM structs

This is more consistent with the way the rest of the driver works and
ensures that all structs we pass into the kernel are zero'd out except for
the fields we actually want to fill.  We were previously doing then when
building with valgrind to keep valgrind from complaining.  However, we need
to start doing this unconditionally as recent kernels have been getting
touchier about this.  In particular, as of kernel commit b31e51360e88 from
Chris Wilson, context creation and destroy fail if the padding bits are not
set to 0.