Avoid redundant superpage promotion attempts on ARM

Because pmap_enter_locked() is called from few different functions
some redundancy in superpage promotion attempts can be observed.
Hence, avoid promotion in pmap_enter_object() (if the object can
be mapped by superpage it will be handled by pmap_enter_object()
itself) and also do not waste time in pmap_enter_quick().
From now on the promotion will be performed only in pmap_enter().
Remove spurious assertion from pmap_extract_locked() on ARM

The condition under assertion is no longer valid since
superpages support is operating on section mappings.
Handle pmap_enter() on already promoted mappings for ARMv6/v7

Attempt to demote the superpage if trying to pmap_enter() on
one. Panic only when the particular superpage should
no longer exist for that pmap and address.