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: 63fba242c464 ("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

drm/meson: add RDMA module driver

The VPU embeds a "Register DMA" that can write a sequence of registers
on the VPU AHB bus, either manually or triggered by an internal IRQ
event like VSYNC or a line input counter.

The initial implementation handles a single channel (over 8), triggered
by the VSYNC irq and does not handle the RDMA irq.

The RDMA will be usefull to reset and program the AFBC decoder unit
on each vsync without involving the interrupt handler that can
be masked for a log period of time, producing display glitches.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021091509.3864-5-narmstrong@baylibre.com