drm/amd/display: fix type mismatch error for return variable

It is possible for ret_vsnprintf to be assigned negative value in
error cases. As an unsigned variable, negative values which
are stored in their 2's complement form gets treated as a positive
number. This will led to treating bad cases as good ones.
eg: -1 gets stored as 0xFFFFFFFF on a 32 bit system

Signed-off-by: Dale Zhao <dale.zhao@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>