amdgpu: Eliminate void* arithmetic in amdgpu_find_bo_by_cpu_mapping

Arithmetic using void* pointers isn't defined by the C standard, only as
a GCC extension. Avoids compiler warnings:

../../amdgpu/amdgpu_bo.c: In function ‘amdgpu_find_bo_by_cpu_mapping’:
../../amdgpu/amdgpu_bo.c:554:48: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
   if (cpu >= bo->cpu_ptr && cpu < (bo->cpu_ptr + bo->alloc_size))
                                                ^
../../amdgpu/amdgpu_bo.c:561:23: warning: pointer of type ‘void *’ used in subtraction [-Wpointer-arith]
   *offset_in_bo = cpu - bo->cpu_ptr;
                       ^

v2: Use uintptr_t instead of char*, don't change function signature
    (Junwei Zhang)

Fixes: 4d454424e1f2 ("amdgpu: add a function to find bo by cpu mapping
                     (v2)")
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>