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>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -4533,6 +4533,7 @@ static void handle_cursor_update(struct drm_plane *plane,
 	amdgpu_crtc->cursor_width = plane->state->crtc_w;
 	amdgpu_crtc->cursor_height = plane->state->crtc_h;
 
+	memset(&attributes, 0, sizeof(attributes));
 	attributes.address.high_part = upper_32_bits(address);
 	attributes.address.low_part  = lower_32_bits(address);
 	attributes.width             = plane->state->crtc_w;