drm/i915: put runtime PM only at the end of intel_mark_idle

drm/i915: put runtime PM only at the end of intel_mark_idle

Because intel_mark_idle still touches some registers: it needs the
machine to be awake. If you set both the autosuspend and PC8 delays to
zero, you can get a "Device suspended" WARN when gen6_rps_idle touches
registers.

This is not easy to reproduce, but happens once in a while when
running pm_pc8.

Testcase: igt/pm_pc8
Signed-off-by: Paulo Zanoni <[email protected]>
Reviewed-by: Jesse Barnes <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>