drm/i915/psr: Always wait for idle state when disabling PSR

It should always wait for idle state when disabling PSR because PSR
could be inactive due a call to intel_psr_exit() and while PSR is
still being disabled asynchronously userspace could change the
modeset causing a call to psr_disable() that will not wait for PSR
idle and then PSR will be enabled again while PSR is still not idle.

v2: rebased on top of the patch reusing psr_exit()

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181106190843.18009-2-jose.souza@intel.com