drm/i915: blow away userspace mappings before fence change

This aligns it with the other user of i915_gem_clear_fence_reg,
which blows away the mapping before changing the fence reg.

Only affects userspace if it races against itself when changing
tiling parameters, i.e. behaviour is undefined, anyway.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>

diff --cc drivers/gpu/drm/i915/i915_gem.c
Simple merge

diff --cc drivers/gpu/drm/i915/i915_gem_tiling.c
Simple merge