drm/amdgpu: Add a parameter to amdgpu_bo_create()

The parameter init_value contains the value to which we initialized
VRAM bo when AMDGPU_GEM_CREATE_VRAM_CLEARED flag is set.

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