drm: mali-dp: Set encoder possible_clones

Set possible_clones field to report that the writeback connector and
the one driving the display could be enabled at the same time.

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>