drm/amdkfd: Use the correct wptr size

commit cdc6705f upstream.

Write pointer could be 32-bit or 64-bit. Use the correct size during
initialization.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>