drm/msm/dp: remove invalid DPCD write

Remove an invalid write operation being made to the read-only
address (byte 0x218) in the DPCD.

CRs-Fixed: 2096493
Change-Id: I3eaa1a317d7f6e98d17e37cc3e743ea8f7e6edeb
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>