drm/amdgpu: use kzalloc for allocating one thing

Use kzalloc rather than kcalloc(1.. for allocating
one thing.

Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Reviewed-by: Vaneet Narang <v.narang@samsung.com>
Reviewed-by: Christian Konig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>