drm/amd/display: Add NULL pointer check for kzalloc

commit 8e65a1b7 upstream.

[Why & How]
Check return pointer of kzalloc before using it.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Wenshan Lan <jetlan9@163.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>