msm: vidc: Fix a race condition in masking interrupts

Fix a race condition when interaction with firmware happens
in following sequence:

1. Firmware writes IDLE message into the message queue.
2. Driver writes a new command into command queue.
3. Firmware reads this command and starts processing it.
4. Driver checks if command queue is empty and finds it empty.
5. Firmware writes the response (of the command sent in 2) in
   message queue. Firmware also writes another IDLE message and
   sets the idle register.
6. Driver check if the IDLE register is set and finds it set.
7. Driver masks the interrupts. Since interrupts are masked,
   driver misses the response written by firmware in 5.

This race condition is fixed by checking if message queue is
empty or not between 6 and 7.

Change-Id: I4aa17cfc5fd9e33f6b41b6c1b12625789ff02283
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>