msm: vidc: fix issue when video session opening failed

There is null pointer access occurring when video session
opening failed and power collapse thread sending sys_error
to all opened sessions in parallel. Fix the issue by doing
v4l2_fh_exit() afer removing new session from core->instances
list.

CRs-Fixed: 1088562
Change-Id: I2523a46487a03ade40afeddd394a4572283d91a2
Signed-off-by: Maheshwar Ajja <majja@codeaurora.org>