drm/amd/display: Remove a false-positive error message

To avoid confusion, need to suppress the error message when get
-ERESTARTSYS error code. It is normal when getting interrupted
by signals in the process of a wait for the buffer to become
unreserved. Only propagate to user-mode for further action,
no need to pop up error message.

Signed-off-by: Jerry Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>