[MIPS] Always use virt_to_phys() when translating kernel addresses

This patch fixes two places where we used plain 'x - PAGE_OFFSET' to
achieve virtual to physical address convertions. This type of convertion
is no more allowed since commit 6f284a2c

.

Reported-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>

[Build fixes for machines that don't use the generic dma-coherence.h]

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>