PM: make VT switching to the suspend console optional v3

PM: make VT switching to the suspend console optional v3

KMS drivers can potentially restore the display configuration without
userspace help.  Such drivers can can call a new funciton,
pm_vt_switch_required(false) if they support this feature.  In that
case, the PM layer won't VT switch to the suspend console at suspend
time and then back to the original VT on resume, but rather leave things
alone for a nicer looking suspend and resume sequence.

v2: make a function so we can handle multiple drivers (Alan)
v3: use a list to track device requests (Rafael)
v4: Squash in build fix from Jesse for CONFIG_VT_CONSOLE_SLEEP=n
v5: Squash in patch from Wu Fengguang to add a few missing static
qualifiers.
v6: Add missing EXPORT_SYMBOL.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> (v3)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>