drm/msm/dp: add dp_debug module

Move debug fs to a new module. Add support to get display
attributes, hot plug connect and disconnect and switch
resolution.

CRs-Fixed: 2094802
Change-Id: I4fe20741c8dfb93b16759facd1e181c6d4ba6838
Signed-off-by: Samantha Tran <samtran@codeaurora.org>