msm: mdss: dp: fix notifications for link training tests

Do not send a connection notification to clients while processing
link training or electrical compliance requests. This is not
necessary as these tests do not require a video output from the
source device. Furthermore, we need to acknowledge/unblock the external
display module if the core and link clocks are already powered on
and we get an power on event. This will allow the driver to make
progress, for example, to send the test responses and requested
audio/video test patterns.

CRs-Fixed: 2006096
Change-Id: I7956248d8ae665290e770e0fd5b76a79c55cda1d
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>