drm/amdgpu: implement TMZ accessor (v3)

Implement an accessor of adev->tmz.enabled. Let not
code around access it as "if (adev->tmz.enabled)"
as the organization may change. Instead...

Recruit "bool amdgpu_is_tmz(adev)" to return
exactly this Boolean value. That is, this function
is now an accessor of an already initialized and
set adev and adev->tmz.

Add "void amdgpu_gmc_tmz_set(adev)" to check and
set adev->gmc.tmz_enabled at initialization
time. After which one uses "bool
amdgpu_is_tmz(adev)" to query whether adev
supports TMZ.

Also, remove circular header file include.

v2: Remove amdgpu_tmz.[ch] as requested.
v3: Move TMZ into GMC.

Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>