msm: VPU: Allow polling for buffers before streaming

Allow VPU clients to poll for buffer events before start of streaming
without returning POLLERR. This is desired because driver might send
asynchronous events like hardware errors at any time, regardless of
streaming state.

Change-Id: Ie17b2e125e90468de122604692388cb93ba84a88
Signed-off-by: Ahmed Abdul-Salam <aabdulsa@codeaurora.org>