drm/i915: Implement stolen reserved detection for ctg/elk

Finally managed to dig up enough hints as to where the stolen
reserved stuff lives on ctg/elk. So add the code to decode it.
This was a combination of old chipset specs, diggin up an old
elk grits release with an ctg/elk AubLoad etc.

This was only tested on an elk as I don't have a ctg here
unfortunately.

This leaves ilk as the only platform that doesn't have a way
to detect this stuff. Looking at the register contents on my
ilk, it might be that the elk way works there too, but I
can't be sure since I can't affect the amount of reserved
memory on that machine, and if I am to trust the register
contents, by default it would reserve 0 bytes.

v2: s/WARN_ON_ONCE/WARN_ON/ since it's in one time init code
    anyway (Paulo)

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index c5287a1..881a47a 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -2494,6 +2494,11 @@
 
 #define MCHBAR_MIRROR_BASE_SNB	0x140000
 
+#define CTG_STOLEN_RESERVED		(MCHBAR_MIRROR_BASE + 0x34)
+#define ELK_STOLEN_RESERVED		(MCHBAR_MIRROR_BASE + 0x48)
+#define G4X_STOLEN_RESERVED_ADDR1_MASK	(0xFFFF << 16)
+#define G4X_STOLEN_RESERVED_ADDR2_MASK	(0xFFF << 4)
+
 /* Memory controller frequency in MCHBAR for Haswell (possible SNB+) */
 #define DCLK (MCHBAR_MIRROR_BASE_SNB + 0x5e04)