drm/i915/skl: Add missing SKL ids

Used by production devices:
    Intel(R) Iris Graphics 540 (Skylake GT3e)
    Intel(R) Iris Graphics 550 (Skylake GT3e)

v2: More ids
v3: Less ids (GT1 got duplicated)

Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454674902-26207-1-git-send-email-michal.winiarski@intel.com