drm/i915: add INTEL_GEN() helper shorthand for INTEL_INFO()->gen

Sudden realization:

$ grep -ho "INTEL_INFO([^)]*)->[a-zA-Z0-9_]*" *.[ch] | sed 's/.*->//' |\
  sort | uniq -c | sort -rn | head -5
  446 gen
   24 num_pipes
   10 ring_mask
    9 color
    4 subslice_per_slice

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460022497-29304-1-git-send-email-jani.nikula@intel.com