MAINTAINERS: fix a few more amdgpu tree links

Switch to gitlab.

Fixes: 101c2fae ("MAINTAINERS: update radeon/amdgpu/amdkfd git trees")
Cc: David Ward <david.ward@gatech.edu>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>