drm/i915: Corrected 'file_priv' to 'file' in 'i915_driver_preclose()'

The 'i915_driver_preclose()' function has a parameter called 'file_priv'.
However, this is misleading as the structure it points to is a 'drm_file' not a
'drm_i915_file_private'. It should be named just 'file' to avoid confusion.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>