drm/i915: more .is_mobile cleanups for BDW

Commit 8d9c20e1 ("drm/i915: Remove .is_mobile field from platform
struct") removed mobile vs desktop differences for HSW+, but forgot
the Broadwell reserved IDs, so do it now.

It's interesting to notice that these IDs are used by early-quirks.c
but are *not* used by i915_pci.c.

Cc: Carlos Santa <carlos.santa@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1483473860-17644-2-git-send-email-paulo.r.zanoni@intel.com