drm: Use max() to make the ioctl alloc size code cleaner

drm: Use max() to make the ioctl alloc size code cleaner

Use max() to make the code to determine the allocation size for
the ioctl data easier to read.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>