drm/amdgpu: simplify padding calculations (v2)

Simplify padding calculations.

v2: Comment update and spacing.

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