drm/i915: Swap magics and use SZ_1M

Since the kernel provides SZ_1M, use it in preference of 1 << 20.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180605135746.8020-1-chris@chris-wilson.co.uk