So from just a quick look we seem to have enough information to
accurately figure out whether a given gem bo is used as a frontbuffer
and where exactly: We have obj->pin_count as a first check with no
false negatives and only negligible false positives. And then we can
just walk the modeset objects and figure out where exactly a buffer is
used as scanout.

Except that we can't due to locking order: If we already hold
dev->struct_mutex we can't acquire any modeset locks, so could
potential chase freed pointers and other evil stuff.

So we need something else. For that introduce a new set of bits
obj->frontbuffer_bits to track where a buffer object is used. That we
can then chase without grabbing any modeset locks.

Of course the consumers of this (DRRS, PSR, FBC, ...) still need to be
able to do their magic both when called from modeset and from gem
code. But that can be easily achieved by adding locks for these
specific subsystems which always nest within either kms or gem
locking.

This patch just adds the relevant update code to all places.

Note that if we ever support multi-planar scanout targets then we need
one frontbuffer tracking bit per attachment point that we expose to
userspace.

v2:
- Fix more oopsen. Oops.
- WARN if we leak obj->frontbuffer_bits when freeing a gem buffer. Fix
  the bugs this brought to light.
- s/update_frontbuffer_bits/update_fb_bits/. More consistent with the
  fb tracking functions (fb for gem object, frontbuffer for raw bits).
  And the function name was way too long.

v3: Size obj->frontbuffer_bits correctly so that all pipes fit in.

v4: Don't update fb bits in set_base on failure. Noticed by Chris.

v5: s/i915_gem_update_fb_bits/i915_gem_track_fb/ Also remove a few
local enum pipe variables which are now no longer needed to make the
function arguments no drop over the 80 char limit.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>