drm/amdgpu: fix incorrect enum type

core_link_write_dpcd() returns enum dc_status, not ddc_result:

display/dc/core/dc_link_dp.c: In function 'dp_set_panel_mode':
display/dc/core/dc_link_dp.c:4237:11: warning: implicit conversion from 'enum dc_status' to 'enum ddc_result'
[-Wenum-conversion]

Avoid the warning by using the correct enum in the caller.

Fixes: 0b226322 ("drm/amd/display: Synchronous DisplayPort Link Training")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>