drm/msm/dp: add support for custom dpcd

Add debug fs support for custom dpcd so that DisplayPort
driver can use a custom dpcd instead of the dpcd read
from the sink. This is needed for debugging and testing
purposes.

CRs-Fixed: 2128730
Change-Id: I61d8b22901a7550932e7870ea06ff1026cc7a13c
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>