drm/i915/gtt: pde entry encoding is identical

For all page directory entries, the pde encoding is
identical. Don't complicate call sites with different
versions of doing the same thing, so we always check the
existence of physical page before writing the entry into
it. This further generalizes the pd so that manipulation in
callsites will be identical, removing the need to handle
pdps differently for gen8.

v2: squash
v3: inc/dec with set/clear (Chris)
v4: inlines, warn, stray set_pd (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190705215204.4559-1-chris@chris-wilson.co.uk