drm/i915/gtt: map the PD up front

We need to generalise our accessor for the page directories and tables from
using the simple kmap_atomic to support local memory, and this setup
must be done on acquisition of the backing storage prior to entering
fence execution contexts. Here we replace the kmap with the object
mapping code that for simple single page shmemfs object will return a
plain kmap, that is then kept for the lifetime of the page directory.

Note that keeping the mapping around is a potential concern here, since
while the vma is pinned the mapping remains there for the PDs
underneath, or at least until the used_count reaches zero, at which
point we can safely destroy the mapping. For 32b this will be even worse
since the address space is more limited, but since this change mostly
impacts full ppGTT platforms, the justification is that for modern
platforms we shouldn't care too much about 32b.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210427085417.120246-3-matthew.auld@intel.com