drm/i915: Use memset64() to align the ring with MI_NOOP

When filling the ring to align the emit pointer to the next cacheline,
use memset64() rather than open-coding it. As we know that we always
have an even number of dwords, we can replace the dword loop with the
qword equivalent.

v2: s/0/MI_NOOP<<32 | MI_NOOP/

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180425123718.16366-1-chris@chris-wilson.co.uk