drm/amd/display: Fix the uninitialized variable in enable_stream_features()

In function enable_stream_features(), the variable "old_downspread.raw"
could be uninitialized if core_link_read_dpcd() fails, however, it is
used in the later if statement, and further, core_link_write_dpcd()
may write random value, which is potentially unsafe.

Fixes: 6016cd9 ("drm/amd/display: add helper for enabling mst stream features")
Cc: stable@vger.kernel.org
Signed-off-by: Yizhuo Zhai <yzhai003@ucr.edu>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Yizhuo Zhai

drivers/gpu/drm/amd/display/dc/core/dc_link.c

@@ -1818,6 +1818,8 @@ static void enable_stream_features(struct pipe_ctx *pipe_ctx)
 		union down_spread_ctrl old_downspread;
 		union down_spread_ctrl new_downspread;
 
+		memset(&old_downspread, 0, sizeof(old_downspread));
+
 		core_link_read_dpcd(link, DP_DOWNSPREAD_CTRL,
 				&old_downspread.raw, sizeof(old_downspread));