drm/i915/bdw: Don't write PDP in the legacy way when using LRCs

This is mostly for correctness so that we know we are running the LR
context correctly (this is, the PDPs are contained inside the context
object).

v2: Move the check to inside the enable PPGTT function. The switch
happens in two places: the legacy context switch (that we won't hit
when Execlists are enabled) and the PPGTT enable, which unfortunately
we need. This would look much nicer if the ppgtt->enable was part of
the ring init, where it logically belongs.

v3: Move the check to the start of the enable PPGTT function.  None
of the legacy PPGTT enabling is required when using LRCs as the
PPGTT is enabled in the context descriptor and the PDPs are written
in the LRC.

v4: Clarify comment based on review feedback.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Thomas Daniel <thomas.daniel@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Resolve conflicts with ppgtt_enable rework.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>