drm/amd/display: remove duplicate assignment of dcn21_funcs members

Fix the following coccicheck warning:

drivers/gpu/drm/amd/display/dc/dcn21/dcn21_init.c:31:51-52:
exit_optimized_pwr_state: first occurrence line 86, second occurrence
line 92
drivers/gpu/drm/amd/display/dc/dcn21/dcn21_init.c:31:51-52:
optimize_pwr_state: first occurrence line 85, second occurrence line 91
drivers/gpu/drm/amd/display/dc/dcn21/dcn21_init.c:31:51-52:
set_cursor_attribute: first occurrence line 71, second occurrence line
89
drivers/gpu/drm/amd/display/dc/dcn21/dcn21_init.c:31:51-52:
set_cursor_position: first occurrence line 70, second occurrence line 88
drivers/gpu/drm/amd/display/dc/dcn21/dcn21_init.c:31:51-52:
set_cursor_sdr_white_level: first occurrence line 72, second occurrence
line 90

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>