drm/amdgpu: improve AMDGPU_GEM_CREATE_VRAM_CLEARED handling (v2)

Drop this whole reserve/unreserve dance, just lock the
reservation object manually when creating the BO.

v2: rebase on dma_fence renaming

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>