disp: msm: dp: add session check for audio register programming

Add a check to ensure that audio registers are programmed only
when the DP timing engine (audio session) is enabled. This will
reduce the likelihood of un-clocked register access for audio
related operations.

Change-Id: I6fe59cf53dc721b5470ad4cf7d84e8606800a246
CRs-Fixed: 2465406
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>