drm/i915: Store platform_mask inside the static device info

Rather than deriving the platform_mask from the
intel_device_static_info->platform at runtime, pre-fill it in the static
data.

v2: Undefine macros at end of their scope

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180215081930.11477-3-chris@chris-wilson.co.uk