drm/i915: Repeat eviction search after idling the GPU

With the advent of hw context support, we gained some objects that are
pinned for the duration of their request. That is we can make aperture
space available by idling the GPU and in the process performing a
context switch back to the always-pinned default context. As such, we
should not conclude that there is no space in the aperture for the
current object until we have unpinned any such context objects.

Note that we also have the problem of outstanding pageflips preventing
eviction of their framebuffer objects to resolve.

Testcase: igt/gem_ctx_exec/eviction
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72507


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: lu hua <huax.lu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>