drm/amdgpu: Cast to uint64_t before left shift

Avoid potential integer overflows with left shift in huge-page mapping
code by casting the operand to uin64_t first.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>