msm: vidc: disable h/w load calculation for thumbnail

Thumbnail decode load should not be considered while
calculating the overall h/w load. This is needed
especially on devices with limited h/w capability.
Without this change, we can see thumbnail decode
failing if two thumbnail sessions start in parallel
and h/w capability is exceeded.

CRs-fixed: 504248
Change-Id: Icdceddc5c15cd5beac391fe15dac9d60e5eb35bb
Signed-off-by: Arun Menon <avmenon@codeaurora.org>