drm/i915: page table abstractions

When we move to dynamic page allocation, keeping page_directory and pagetabs as
separate structures will help to break actions into simpler tasks.

To help transition the code nicely there is some wasted space in gen6/7.
This will be ameliorated shortly.

Following the x86 pagetable terminology:
PDPE = struct i915_page_directory_pointer_entry.
PDE = struct i915_page_directory_entry [page_directory].
PTE = struct i915_page_table_entry [page_tables].

v2: fixed mismatches after clean-up/rebase.

v3: Clarify the names of the multiple levels of page tables (Daniel)

v4: Addressing Mika's review comments.
s/gen8_free_page_directories/gen8_free_page_directory and free the
page tables for the directory there.
In gen8_ppgtt_allocate_page_directories, do not leak previously allocated
pt in case the page_directory alloc fails.
Update error return handling in gen8_ppgtt_alloc.

v5: Do not leak pt on error in gen6_ppgtt_allocate_page_tables. (Mika)

v6: s/page_tables/page_table/. (Mika)

Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Michel Thierry <michel.thierry@intel.com> (v2+)
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.h

@@ -187,6 +187,20 @@ struct i915_vma {
 			 u32 flags);
 };
 
+struct i915_page_table_entry {
+	struct page *page;
+};
+
+struct i915_page_directory_entry {
+	struct page *page; /* NULL for GEN6-GEN7 */
+	struct i915_page_table_entry *page_table;
+};
+
+struct i915_page_directory_pointer_entry {
+	/* struct page *page; */
+	struct i915_page_directory_entry page_directory[GEN8_LEGACY_PDPES];
+};
+
 struct i915_address_space {
 	struct drm_mm mm;
 	struct drm_device *dev;
@@ -271,11 +285,6 @@ struct i915_hw_ppgtt {
 	struct drm_mm_node node;
 	unsigned num_pd_entries;
 	unsigned num_pd_pages; /* gen8+ */
-	union {
-		struct page **pt_pages;
-		struct page **gen8_pt_pages[GEN8_LEGACY_PDPES];
-	};
-	struct page *pd_pages;
 	union {
 		uint32_t pd_offset;
 		dma_addr_t pd_dma_addr[GEN8_LEGACY_PDPES];
@@ -284,6 +293,10 @@ struct i915_hw_ppgtt {
 		dma_addr_t *pt_dma_addr;
 		dma_addr_t *gen8_pt_dma_addr[GEN8_LEGACY_PDPES];
 	};
+	union {
+		struct i915_page_directory_pointer_entry pdp;
+		struct i915_page_directory_entry pd;
+	};
 
 	struct drm_i915_file_private *file_priv;