drm/amdgpu: revert "Add support for filling a buffer with 64 bit value"

This reverts commit 7bdc53f9 and commit
330df03b.

Neither are needed any more.

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