drm: Define a drm_invalid_op ioctl implementation

And use it in radeon to replace all the ioctls no longer valid in kms
mode. I plan to also use this later on when nuking the ums support for
i915.

Note that setting the function pointer in the ioctl table to NULL
would amount to the same, but that results in some debug output from
the drm_ioctl() function. I've figured it's cleaner to have a
special-purpose function.

Cc: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>