msm: vidc: Drop Frame logic

Change the filled length of the buffer to 0 with
BUFFERFLAG_DROP_FRAME & BUFFERFLAG_DECODE_ONLY set so that the
driver's client can resubmit the buffers instead of sending to
display.

CRs-Fixed: 2016195
Change-Id: I294b14b45391e8bf620afc0b19cdf64ef690fd20
Signed-off-by: Umesh Pandey <umeshp@codeaurora.org>