drm/i915: Use dev_priv as first argument of for_each_pipe()

Chris has decided that enough is enough. It's time to fixup dev Vs
dev_priv. This is a modest contribution to the crusade.

v2: Still use INTEL_INFO(), for the (mythical!) case we want to hardcode
    the info struct with defines (Chris)
    Rename the macro argument from 'dev' to 'dev_priv' (Jani)

v3: Use names unlikely to be used as macro arguments (Chris)

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>