drm/amdkfd: Remove BUG_ONs for NULL pointer arguments

Remove BUG_ONs that check for NULL pointer arguments that are
dereferenced in the same function. Dereferencing the NULL pointer
will generate a BUG anyway, so the explicit check is redundant and
unnecessary overhead.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>