MAINTAINERS: Update mailing list for radeon and amdgpu

amdgpu and radeon development has moved to this list.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

MAINTAINERS

@@ -4116,7 +4116,7 @@ F:	drivers/gpu/drm/cirrus/
 RADEON and AMDGPU DRM DRIVERS
 M:	Alex Deucher <alexander.deucher@amd.com>
 M:	Christian König <christian.koenig@amd.com>
-L:	dri-devel@lists.freedesktop.org
+L:	amd-gfx@lists.freedesktop.org
 T:	git git://people.freedesktop.org/~agd5f/linux
 S:	Supported
 F:	drivers/gpu/drm/radeon/