drm/msm/dp: add support for custom edid

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

CRs-Fixed: 2128730
Change-Id: I2b50c4ae857bea568b7c1d65fc8eaffd3521b8bc
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>