msm: vidc: add control to enable Hierarchical P num layers

This change will enable firmware to perform Hierarchical
video encoding for P frames. Hierarchical video encoding is
used to improve error resilience and temporal scalability.
The encoded frames are logically organized in to multiple
layers with frames in one layer referencing only frames
from lower layers. The lowest layer, i.e. base layer, is
the only exception.

Change-Id: Ie30e6d075b76b3337ec895584cf1f3be9f4bb6c6
Signed-off-by: Arun Menon <avmenon@codeaurora.org>