disp: msm: dp: avoid setting multi_func when pin value is zero

During dp altmode driver initialization, the dp altmode driver is called
with a null data packet. Even though this callback indicates a disconnect,
the multi function flag is incorrectly set to true. If the DP is enabled
later using simulation mode, the multi function flag is not cleared thereby
limiting the lane count to 2 irrespective of capability applied through
DPCD.

This change checks for the zero pin value while setting the multi function
flag.

Change-Id: I974fbf1630eed26f406ac9e4c99140b27f0cb635
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>