drm/amdgpu: remove pointless ttm_eu usage from DM

We just need to reserve one BO here, no need for using ttm_eu to reserve
multiple BOs.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220419141915.122157-2-christian.koenig@amd.com