drm/i915: Enlarge vma->pin_count

Previously we only accommodated having a vma pinned by a small number of
users, with the maximum being pinned for use by the display engine. As
such, we used a small bitfield only large enough to allow the vma to
be pinned twice (for back/front buffers) in each scanout plane. Keeping
the maximum permissible pin_count small allows us to quickly catch a
potential leak. However, as we want to split a 4096B page into 64
different cachelines and pin each cacheline for use by a different
timeline, we will exceed the current maximum permissible vma->pin_count
and so time has come to enlarge it.

Whilst we are here, try to pull together the similar bits:

Address/layout specification:
 - bias, mappable, zone_4g: address limit specifiers
 - fixed: address override, limits still apply though
 - high: not strictly an address limit, but an address direction to search

Search controls:
 - nonblock, nonfault, noevict

v2: Rewrite the guideline comment on bit consumption.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: John Harrison <john.C.Harrison@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190128181812.22804-2-chris@chris-wilson.co.uk