drm/i915: Eliminate nested get/put pages

By using read_cache_page() for individual pages during pwrite/pread we
can eliminate an unnecessary large allocation (and immediate free) of
obj->pages. Also this eliminates any potential nesting of get/put pages,
simplifying the code and preparing the path for greater things.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_drv.h

@@ -747,15 +747,6 @@ struct drm_i915_gem_object {
 	 */
 	unsigned int madv : 2;
 
-	/**
-	 * Refcount for the pages array. With the current locking scheme, there
-	 * are at most two concurrent users: Binding a bo to the gtt and
-	 * pwrite/pread using physical addresses. So two bits for a maximum
-	 * of two users are enough.
-	 */
-	unsigned int pages_refcount : 2;
-#define DRM_I915_GEM_OBJECT_MAX_PAGES_REFCOUNT 0x3
-
 	/**
 	 * Current tiling mode for the object.
 	 */