MAINTAINERS: Update AMDGPU DML maintainers info

Chaitanya is no longer with AMD, and the responsibility has been
taken over by Austin.

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>