drm/i915: Move intel_device_info definitions to its own header

We already keep intel_device_info functions in dedicated file.
Add matching header file and move related definitions there.

v2: add gen boundaries (Chris)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171221185334.17396-5-michal.wajdeczko@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171221215735.30314-4-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/intel_device_info.c b/drivers/gpu/drm/i915/intel_device_info.c
index be355cf..f205054 100644
--- a/drivers/gpu/drm/i915/intel_device_info.c
+++ b/drivers/gpu/drm/i915/intel_device_info.c
@@ -24,6 +24,7 @@
 
 #include <drm/drm_print.h>
 
+#include "intel_device_info.h"
 #include "i915_drv.h"
 
 #define PLATFORM_NAME(x) [INTEL_##x] = #x