drm/i915: move psr_setup_done to psr struct

drm/i915: move psr_setup_done to psr struct

"Because our driver assumes only one panel is PSR capable, and we
already have other PSR information on dev_priv instead of intel_dp. If
we ever support multiple PSR panels, we'll have to move struct
i915_psr to intel_dp anyway." (by Paulo)

v2: Avoid more than one setup. Removing initialization
    and trusting allocation. (By Paulo Zanoni).
v3: rebase.
v4: Adding comment.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>