All apart from AMDGPU_GEM_DOMAIN_GTT memory domains map 1:1 to TTM
placements. And the former be either AMDGPU_PL_PREEMPT or TTM_PL_TT,
depending on AMDGPU_GEM_CREATE_PREEMPTIBLE.

Simplify a few places in the code which convert the TTM placement into
a domain by checking against the current placement directly.

In the conversion AMDGPU_PL_PREEMPT either does not have to be handled
because amdgpu_mem_type_to_domain() cannot return that value anyway.

v2:
 * Remove AMDGPU_PL_PREEMPT handling.

v3:
 * Rebase.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Christian König <christian.koenig@amd.com> # v1
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com> # v2
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>