drm/amdgpu: Workaround build failure due to trace conflict

Avoid including mmu_context.h in amdgpu_amdkfd.h since that may be
included in other header files that define traces. This leads to
conflicts due to traces defined in other headers included via
mmu_context.h.

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