drm/amdgpu: fix 32-bit build warning

Casting a pointer to a 64-bit type causes a warning on 32-bit targets:

drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gfx_v9.c:473:24: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
          lower_32_bits((uint64_t)wptr));
                        ^
drivers/gpu/drm/amd/amdgpu/amdgpu.h:1701:53: note: in definition of macro 'WREG32'
 #define WREG32(reg, v) amdgpu_mm_wreg(adev, (reg), (v), 0)
                                                     ^
drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gfx_v9.c:473:10: note: in expansion of macro 'lower_32_bits'
          lower_32_bits((uint64_t)wptr));
          ^~~~~~~~~~~~~

The correct method is to cast to 'uintptr_t'.

Fixes: d5a114a6

 ("drm/amdgpu: Add GFXv9 kfd2kgd interface functions")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>