drm: Rewrite drm_ioctl_flags() to resemble the new drm_ioctl() code

Use the same logic when checking for valid ioctl range in
drm_ioctl_flags() that is used in drm_ioctl() to avoid
confusion.

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