drm/msm/dp: check for downstream devices before reading edid

Branch devices may not have a downstream device connected.
In those cases, branch devices may indicate absence of
connected devices. Do not read EDID in case downstream
devices are not connected to avoid EDID read failures.

CRs-Fixed: 2159660
Change-Id: I1f71e4c95bcb3c3c11df6f0890d98766c3962dc4
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>