MAINTAINERS: Remove unneded wildcard for the Radeon/AMDGPU drivers

There are no other files but the UAPI headers, thus we can drop the
wildcard.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>