msm: vidc: Update minimum GOP size calculation

Use max layers allowed in a session to calculate min GOP size
instead of variable number of layers enabled during each frame
encode to avoid GOP size not being multiple of sub-GOP size issue.

Change-Id: I3bb6e5320385acea9cb892c04b586ac3ca8c753f
Signed-off-by: Akshata Sahukar <asahukar@codeaurora.org>