drm/i915/bxt: Add DC9 Trigger sequence

Add triggers for DC9 as per details provided in bxt_enable_dc9
and bxt_disable_dc9 implementations.

v1:
- Add SKL check in gen9_disable_dc5 as it is possible for DC5
  to remain disabled only for SKL.
- Add additional checks for whether DC5 is already disabled during
  DC5-disabling only for BXT.

v2:
- rebase to latest.
- Load CSR during DC9 disabling in the beginning before DC9 is
  disabled.
- Make gen9_disable_dc5 function non-static as it's being called by
  functions in i915_drv.c.
- Enable DC9-related functionality using a macro.

v3: (imre)
- remove BXT_ENABLE_DC9, we want DC9 always, and it's only valid on BXT
- remove DC5 disabling and CSR FW loaded check, these are nop atm
- squash in Vandana's "Do ddi_phy_init always" patch

v4:
- add TODO to re-enable DC5 during resume if CSR FW is available (sagar)

Signed-off-by: Suketu Shah <suketu.j.shah@intel.com>
Signed-off-by: A.Sunil Kamath <sunil.kamath@intel.com> (v2)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sagar Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>