xlat v2: Correctly unmap regions on map error

xlat v2: Correctly unmap regions on map error

`mm_cursor` doesn't have the needed data because the `memmove()` that
is called right before it overwrites that information. In order to get
the information of the region that was being mapped, `mm` has to be used
instead (like it is done to fill the fields of `unmap_mm`).

If the incorrect information is read, this check isn't reliable and
`xlat_tables_unmap_region` may be requested to unmap memory that isn't
mapped at all, triggering assertions.

Change-Id: I602d4ac83095d4e5dac9deb34aa5d00d00e6c289
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>