msm: vidc: remove additional checks in response_handler

possibility of OOB access on device->response_pkt in
__response_handler. for e.x if msg queue contains 1000
messages and all 1000 were read and queue is empty.
So __get_q_size api will return zero and _iface_msgq_read
will go in an infinite loop, even if packet_count
== max_packets.

Change-Id: I72c26f68e4c027572ee496ade3c9a6828f865e74
Signed-off-by: Vasantha Balla <vballa@codeaurora.org>