drivers: staging: sm750: Hold lock irrespective of fb numbers.

Start holding the lock for all cases irrespective of number of fb,
there could be a deadlock since this number could change in the
lifetime of this lock

Signed-off-by: Dhananjay Balan <mail@dbalan.in>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>