drm/dp: Implement I2C_M_STOP for i2c-over-aux

drm/dp: Implement I2C_M_STOP for i2c-over-aux

Consult the I2C_M_STOP flag to determine whether to set the MOT bit or
not. Makes it possible to send multiple messages in one go with
stop+start generated between the messages (as opposed nothing or
repstart depending on whether thr address/rw changed).

Not sure anyone has actual use for this but figured I'd handle it
since I started to look at that flag for MST remote i2c xfers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180928180403.22499-3-ville.syrjala@linux.intel.com
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>