drm/dp_mst: correct the shifting in DP_REMOTE_I2C_READ

drm/dp_mst: correct the shifting in DP_REMOTE_I2C_READ

[Why]
According to DP spec, it should shift left 4 digits for NO_STOP_BIT
in REMOTE_I2C_READ message. Not 5 digits.

In current code, NO_STOP_BIT is always set to zero which means I2C
master is always generating a I2C stop at the end of each I2C write
transaction while handling REMOTE_I2C_READ sideband message. This issue
might have the generated I2C signal not meeting the requirement. Take
random read in I2C for instance, I2C master should generate a repeat
start to start to read data after writing the read address. This issue
will cause the I2C master to generate a stop-start rather than a
re-start which is not expected in I2C random read.

[How]
Correct the shifting value of NO_STOP_BIT for DP_REMOTE_I2C_READ case in
drm_dp_encode_sideband_req().

Changes since v1:(https://patchwork.kernel.org/patch/11312667/)
* Add more descriptions in commit and cc to stable

Fixes: ad7f8a1f9ced ("drm/helper: add Displayport multi-stream helper (v0.6)")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200103055001.10287-1-Wayne.Lin@amd.com