drm/i915: Add IS_PLATFORM macro

This will allow some code re-organization in a following patch.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170920092701.17963-2-tvrtko.ursulin@linux.intel.com