drm/i915: Create a common GEN9_LP_FEATURE.

The following LP platform inherits a lot of this platform
So let's simplify here to re-use this later.

v2: Keep ddb_size out of the new macro.
v3: Rebase (has_decoupled_mmio). (Imre)

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480584796-19466-1-git-send-email-ander.conselvan.de.oliveira@intel.com