drm: don't override possible_crtcs for primary/cursor planes

It is kind of a pointless restriction.  If userspace does silly things
like using crtcA's cursor plane on crtcB, and then setcursor on crtcA,
it will end up with the overlay disabled on crtcB.  But userspace is
allowed to shoot itself like this.

v2: don't WARN_ON() if caller did not set ->possible_crtcs.  This keeps
    the existing behavior by default, if caller does not set the
    ->possible_crtcs.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1478357521-26542-1-git-send-email-robdclark@gmail.com