drm/i915: Factor out setup_private_pat()

drm/i915: Factor out setup_private_pat()

Factor out setup_private_pat() for introducing the following patches.

Signed-off-by: Zhi Wang <[email protected]>
Cc: Ben Widawsky <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Reviewed-by: Ben Widawsky <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Chris Wilson <[email protected]>