drm/i915: Convert i915_gem_object_ops->flags values to use BIT()

Having just watched someone add a new value, 0x3, without realising that
the flags were bit values, I have come to appreciate the value in using
BIT.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170523103116.32239-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>