drm/amdgpu: fix size calculation with stolen vga memory

If we need to keep the stolen vga memory, make sure it is
at least as big as the legacy vga size.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org