drm/dp: complete the implementation for remote dpcd read

Current DP MST topology layer has incomplete implementation
for drm_dp_send_dpcd_read function. This change completes
the implementation in order to make it ready for exporting
the function to the dp mst drivers.

Change-Id: I59ce468d0353dab54b37bee7d6917d15cad16020
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>