msm: vidc: Increase the MIN DRAIN RATE when DCVS is enabled

This change increases the MIN_DRAIN_RATE from 1 to 2 initially
and the low threshold gets calculated based on drain rate later on.
It also includes the fix to avoid updating the FTB statistics for
output buffers with zero filled length in case of port reconfiguration.

Change-Id: Ia9065e1d761e0cc944091a1dd21ebcfb0cd8a5a8
Signed-off-by: Sowmya Pandiri <spandiri@codeaurora.org>