msm: vidc: Don't query the V4L2 ctrl framework for checking thumbnail mode

The V4L2 control framework is not re-entrant.  As a result if we attempt
to call g_ctrl() within the context of s_ctrl(), we end up deadlocking.
To prevent calling g_ctrl() store the state of thumbnail mode as a
instance flag.

Change-Id: I84dd024f4c23729f211646450a0dcf91eb6efa3e
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>