msm: vidc: Check if buffers are mapped before validating buffer_info

When buffers are not mapped by the driver (i.e. buffer being used by
kernel client), inst->registered_buffers is usually empty.  As a result,
in *_dqbuf(), binfo is always NULL.  This commit bypasses the NULL check
if buffers aren't being mapped by the driver.

CRs-Fixed: 610866
Change-Id: I111150564549a74f077ba8a7115129f497d734fc
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>