drm/msm/dp: exit power saving mode before link training

Exit power saving mode before link training to ensure that
the sink is powered on and responsive once link training
starts.

CRs-Fixed: 2334519
Change-Id: I062215c1bcd241784a0eeeb25ae07e24e0a2811b
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>