drm/msm/dp: Fix DP stream count if DP MST is disabled

Even when the DP MST property is not present in the DT file, the dp
driver is returning the number of streams as 2. Correcting the code
to return number of streams as 0 when DP MST is not supported.

Change-Id: I7c41d8739698e265776d94931bbcfe7212f96509
Signed-off-by: Sankeerth Billakanti <sbillaka@codeaurora.org>