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>