drm/i915: Add missing description to parameter in alloc_pt_range

The patch "drm/i915: Plumb drm_device through page tables operations"
added an extra parameter, but it didn't update the function description.
Also remove unnecessary blank line added by the same patch.

Found by kbuild test robot.
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -309,6 +309,7 @@ static struct i915_page_table_entry *alloc_pt_single(struct drm_device *dev)
  *		available to point to the allocated page tables.
  * @pde:	First page directory entry for which we are allocating.
  * @count:	Number of pages to allocate.
+ * @dev:	DRM device.
  *
  * Allocates multiple page table pages and sets the appropriate entries in the
  * page table structure within the page directory. Function cleans up after
@@ -318,7 +319,6 @@ static struct i915_page_table_entry *alloc_pt_single(struct drm_device *dev)
  */
 static int alloc_pt_range(struct i915_page_directory_entry *pd, uint16_t pde, size_t count,
 		  struct drm_device *dev)
-
 {
 	int i, ret;