drm/i915/psr: Fix warning in intel_psr_activate()

drm/i915/psr: Fix warning in intel_psr_activate()

commit 5422b37c907e ("drm/i915/psr: Kill delays when activating psr
back.") removed the call to cancel a scheduled psr_work from
psr_disable() and instead added an early return in the work function. But,
if the scheduled work item is executed after psr_enable(), we end up
printing warnings as PSR is already enabled and active. So, put the
cancel_work call back in psr_disable().

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Fixes: 5422b37c907e ("drm/i915/psr: Kill delays when activating psr back.")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106948
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180618220207.2778-1-dhinakaran.pandiyan@intel.com