drm/i915/skl: Making DC6 entry is the last call in suspend flow.

Mmio register access after dc6/dc5 entry is not allowed when
DC6 power states are enabled according to bspec (bspec-id 0527),
so enabling dc6 as the last call in suspend flow.

Addtional note from Imre:

Currently we keep DC6 enabled during modesets and DPAUX transfers, which
is not allowed according to the specification. This can lead at least to
PLL locking failures, DPAUX timeouts and prevent deeper package power
states (PC9/10). Fix this for now by enabling DC6 only when we know the
above events (modeset, DPAUX) can't happen.

This a temporary solution as some issues are still unsolved as described
in [1] and [2], we'll address those as a follow-up.

[1]
http://lists.freedesktop.org/archives/intel-gfx/2015-October/077669.html
[2]
http://lists.freedesktop.org/archives/intel-gfx/2015-October/077787.html



v1: Initial version.

v2: Based on review comment from Daniel,
- created a seperate patch for csr uninitialization set call.

v3: Rebased on top of latest code.

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Sunil Kamath <sunil.kamath@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@intel.com>
Acked-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>