drm/amdgpu: fix seq_printf format string

drm/amdgpu: fix seq_printf format string

The amdgpu driver has a debugfs interface that shows the amount of
VRAM in use, but the newly added code causes a build error on
all 32-bit architectures:

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c:1076:17: warning: format '%lu' expects argument of type 'long unsigned int', but argument 4 has type 'long long int' [-Wformat=]

This fixes the format string to use "%llu" for printing 64-bit
numbers, which works everywhere, as long as we also cast to 'u64'.
Unlike atomic64_t, u64 is defined as 'unsigned long long' on
all architectures.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: a2ef8a974931 ("drm/amdgpu: add vram usage into debugfs")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>