drm/amd/display: fix cursor black issue

[Why]
the member sdr_white_level of struct dc_cursor_attributes was not
initialized, then the random value result that
dcn10_set_cursor_sdr_white_level() set error hw_scale value 0x20D9(normal
value is 0x3c00), this cause the black cursor issue.

[how]
just initilize the obj of struct dc_cursor_attributes to zero to avoid
the random value.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Tianci Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>