drm/i915: Code cleanup to use dev_priv and INTEL_GEN

Replace dev with dev_priv and INTEL_INFO with INTEL_GEN

v1:
* Rebased on drm-nightly (Jani Nikula)
* Separated from the link training patch series

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475019413-19811-1-git-send-email-manasi.d.navare@intel.com