drm/i915: Remove useless VLV_FEATURE Macro.

This macro got useless after commit 8d9c20e1
"drm/i915: Remove .is_mobile field from platform struct"

that removed is_mobile split from VLV definition.
Also this was never reused on any following platform.

So let's clean up a bit here.

Cc: Carlos Santa <carlos.santa@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1482184508-18346-1-git-send-email-rodrigo.vivi@intel.com