MAINTAINERS: Add Xinhui Pan as another AMDGPU contact

Since Chunming Zhou left AMD last year we are down to only
two maintainers once more. So add Xinhu Pan as another
contact as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Xinhui Pan <xinhui.pan@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210505110146.11689-1-christian.koenig@amd.com