drm: setclientcap doesn't need the drm BKL

It only updates per-file feature flags. And all the ioctl which change
behaviour depending upon these flags (they're all kms features) do
_not_ hold the BKL. Therefor this is pure cargo-cult and can be
removed.

Note that there's a risk that the ioctl will behave inconsistently
when userspace is racing with itself, but that's ok. The only thing
it's not allowed to do is oops the kernel, and from an audit all
places are safe.

v2: Clarify that the inconsistency is only when userspace races
(Chris).

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161210215255.7765-2-daniel.vetter@ffwll.ch