drm/i915/glk: Add a IS_GEN9_LP() macro

Broxton and Geminilake are both gen9lp platforms. To avoid adding
IS_GEMINILAKE() checks everywhere alongside the IS_BROXTON() ones, add a
IS_GEN9_LP() macro.

v2: Rename macro parameter to dev_priv. (Joonas)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>