drm/i915/icl: Enable DC9 as lowest possible state during screen-off

ICL supports DC5, DC6, and DC9. Enable DC9 during screen-off, and enable
DC5/6 when appropriate.

v2: (James Ausmus)
 - Also handle ICL as GEN9_LP in i915_drm_suspend_late and
   i915_drm_suspend_early
 - Add DC9 to gen9_dc_mask for ICL
 - Re-order GEN checks for newest platform first
 - Use INTEL_GEN instead of INTEL_INFO->gen
 - Use INTEL_GEN >= 11 instead of IS_ICELAKE
 - Consolidate GEN checks

v3: (James Ausmus)
 - Also allow DC6 for ICL (Imre, Art)
 - Simplify !(GEN >= 11) to GEN < 11 (Imre)

v4: (James Ausmus)
 - Don't call intel_power_sequencer_reset after DC9 for Gen11+, as the
   PPS regs are Always On
 - Rebase against upstream changes

v5: (Anusha Srivatsa)
- rebased against the latest upstream changes.

v6: (Anusha Srivatsa)
- rebased.Use INTEL_GEN consistently.
- Simplify the code (Rodrigo)

v7: rebased. Change order according to platforms(Jyoti)

v8: rebased. Change the check from platform specific to
HAS_PCH_SPLIT(). Add comment in code to be more clear.(Rodrigo)

Cc: Imre Deak <imre.deak@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Tested-by: Jyoti Yadav <jyoti.r.yadav@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181029221410.4423-1-anusha.srivatsa@intel.com