vidc: Add check for ION import fd errors.

ION APIs return error values rather than NULL pointers in case of
failure.  Hence remove the explicit NULL check and use the IS_ERR macro
to identify failure.

Change-Id: I38d6ce06e676dfc0f85954bf509db53295906bbe
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>