drm/i915/trace: Context field needs to be 64-bit wide

Underlaying field is u64 so the tracepoint needs to be as well.

v2:
 * Re-order binary packet for 64-bit alignment. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180605134124.25672-1-tvrtko.ursulin@linux.intel.com