drm/i915: Remove _entry from PPGTT page structures

Lets try to keep this consistent:

Page Directory Pointer (PDP).
Page Directory (PD), also known as page directory pointer entries.
Page Table (PT), also known as page directory entries.

s/struct i915_page_table_entry/struct i915_page_table/
s/struct i915_page_directory_entry/struct i915_page_directory/
s/struct i915_page_directory_pointer_entry/struct
i915_page_directory_pointer/

Suggested-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>