drm/amd/powerplay: avoid NULL pointer dereference

if (a == NULL || a->b == NULL)
leads to a NULL pointer dereference if a == NULL.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>