drm/imx: Remove the bogus possible_clones setup

It's not at all clear what cloning options this driver supports.
So let's just clear possible_clones instead of setting it to some
bogus value.

v2: Adjust the FIXME (Daniel)

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211162208.16224-5-ville.syrjala@linux.intel.com