drm/amdgpu: use %pad format string for dma_addr_t

DMA addresses can be shorter than u64, which results in a broken debug output:

drivers/gpu/drm/amd/amdgpu/amdgpu_gart.c: In function 'amdgpu_gart_table_ram_alloc':
drivers/gpu/drm/amd/amdgpu/amdgpu.h:41:22: error: format '%llx' expects argument of type 'long long unsigned int', but argument 4 has type 'dma_addr_t' {aka 'unsigned int'} [-Werror=format=]
drivers/gpu/drm/amd/amdgpu/amdgpu_gart.c:146:9: note: in expansion of macro 'dev_info'
  146 |         dev_info(adev->dev, "%s dma_addr:%llx\n", __func__, dma_addr);

Use the special %pad format string and pass the DMA address by reference.

Fixes: c9a502e9 ("drm/amdgpu: Allocate GART table in RAM for AMD APU")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>