MAINTAINERS: Fix TTM tree

TTM uses drm-misc now.  Update the tree.

Cc: David Ward <david.ward@gatech.edu>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210503134736.1467003-1-alexander.deucher@amd.com