drm/amd/amdgpu: cast mem->num_pages to 64-bits when shifting (v2)

On 32-bit hosts mem->num_pages is 32-bits and can overflow
when shifted.  Add a cast to avoid this.

(v2): Style fix.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>