MAINTAINERS: .mailmap: update Rob Clark's email address

Remap historical email addresses to @oss.qualcomm.com.

Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Acked-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Patchwork: https://patchwork.freedesktop.org/patch/656974/