drm/i915: use platform enum instead of duplicating PCI ID if possible

Duplicating the PCI ID for IS_FOO checks is redundant for a bunch of
platforms. Simplify.

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/4f79321aca2e003a627ba8b6809af3716b7c25c9.1480520526.git.jani.nikula@intel.com