drm/dp/mst: Provide defines for ACK vs. NAK reply type

Make the code a bit easier to read by providing symbolic names
for the reply_type (ACK vs. NAK). Also clean up some brace stuff
while at it.

v2: s/DP_REPLY/DP_SIDEBAND_REPLY/ (DK)
    Fix some checkpatch issues

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190122200301.18633-1-ville.syrjala@linux.intel.com