drm/i915: Pass dev_priv to INTEL_INFO everywhere apart from the gen use

After this patch only conversion of INTEL_INFO(p)->gen to
INTEL_GEN(dev_priv) remains before the __I915__ macro can
be removed.

v2: Tidy vlv_compute_wm. (David Weinehall)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>