drm: sun4i: print DMA address correctly

The newly added sun4i drm driver prints a dma address using the %x
format string, which cannot work when dma_addr_t is 64 bit,
and gcc warns about this configuration:

drm/sun4i/sun4i_backend.c: In function 'sun4i_backend_update_layer_buffer':
drm/sun4i/sun4i_backend.c:193:84: error: format '%x' expects argument of type 'unsigned int', but argument 3 has type 'dma_addr_t {aka long long unsigned int}' [-Werror=format=]
  DRM_DEBUG_DRIVER("Using GEM @ 0x%x\n", gem->paddr);
drm/sun4i/sun4i_backend.c:201:84: error: format '%x' expects argument of type 'unsigned int', but argument 3 has type 'dma_addr_t {aka long long unsigned int}' [-Werror=format=]
  DRM_DEBUG_DRIVER("Setting buffer address to 0x%x\n", paddr);

This changes the code to use the explicit %pad format string, which
always prints the right length.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>