drm/amdgpu: make AMDGPU_VA_RESERVED_SIZE 64bit

Even when it's a small handle it as 64bit value as well.

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