drm: meson: fix address type confusion

Casting a pointer to dma_addr_t produces a warning:

drivers/gpu/drm/meson/meson_rdma.c: In function 'meson_rdma_free':
drivers/gpu/drm/meson/meson_rdma.c:59:25: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
  priv->rdma.addr_phys = (dma_addr_t)NULL;

In this case, it's worse because the variable name has the suffix
'_phys', which often indicates a phys_addr_t rather than dma_addr_t,
i.e. yet another incompatible type.

Change it to use consistent naming and avoid NULL.

Fixes: 63fba242

 ("drm/meson: add RDMA module driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200107214653.1173199-1-arnd@arndb.de