Memory: Fix crash when unmapping a VMA covering cached surfaces

Unmapping pages tries to flush any cached GPU surfaces touching that
region. When a cached page is invalidated, GetPointerFromVMA() is used
to restore the original pagetable pointer. However, since that VMA has
already been deleted, this hits an UNREACHABLE case in that function.

Now when this happens, just set the page type to Unmapped and continue,
which arrives at the correct end result.