amdgpu: fix 32-on-64 support (v2)

0xffffffff was appended to the higher 32bit with (intptr_t) when use
32bit libdrm_amdgpu.so with 64bit kernel, and it caused segmentation
fault for 32bit application.

v2: switch to uintptr_t

Signed-off-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>