drm/i915: Use phys_addr_t for the address of stolen memory

Though we know the hw is limited to keeping stolen memory inside the
first 4GiB, it is clearer to the reader that we are handling physical
address if we use phys_addr_t to refer to the base of stolen memory.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170106152013.24684-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -1453,7 +1453,7 @@ struct i915_gem_mm {
 	struct work_struct free_work;
 
 	/** Usable portion of the GTT for GEM */
-	unsigned long stolen_base; /* limited to low memory (32-bit) */
+	phys_addr_t stolen_base; /* limited to low memory (32-bit) */
 
 	/** PPGTT used for aliasing the PPGTT with the GTT */
 	struct i915_hw_ppgtt *aliasing_ppgtt;