drm/i915: Extend LRC pinning to cover GPU context writeback

Use the first retired request on a new context to unpin
the old context. This ensures that the hw context remains
bound until it has been written back to by the GPU.
Now that the context is pinned until later in the request/context
lifecycle, it no longer needs to be pinned from context_queue to
retire_requests.
This fixes an issue with GuC submission where the GPU might not
have finished writing back the context before it is unpinned. This
results in a GPU hang.

v2: Moved the new pin to cover GuC submission (Alex Dai)
    Moved the new unpin to request_retire to fix coverage leak
v3: Added switch to default context if freeing a still pinned
    context just in case the hw was actually still using it
v4: Unwrapped context unpin to allow calling without a request
v5: Only create a switch to idle context if the ring doesn't
    already have a request pending on it (Alex Dai)
    Rename unsaved to dirty to avoid double negatives (Dave Gordon)
    Changed _no_req postfix to __ prefix for consistency (Dave Gordon)
    Split out per engine cleanup from context_free as it
    was getting unwieldy
    Corrected locking (Dave Gordon)
v6: Removed some bikeshedding (Mika Kuoppala)
    Added explanation of the GuC hang that this fixes (Daniel Vetter)
v7: Removed extra per request pinning from ring reset code (Alex Dai)
    Added forced ring unpin/clean in error case in context free (Alex Dai)

Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Issue: VIZ-4277
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Gordon <david.s.gordon@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Alex Dai <yu.dai@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Alex Dai <yu.dai@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>