msm: vidc: Reset Venus firmware cookie during failure

When there is an error while loading Venus firmware, the
cookie tracking the firmware is not reset to NULL. Due
to this, there is no further attempt to download the
firmware for any subsequent video session. Unless the
device is rebooted, firmware fails to download and hence
video session continue to fail.
Reset the cookie to NULL to attempt firmware download
during subsequent video session.

Change-Id: I042998a89352894b3d8ee171d3001d7cba33b818
Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>