PM: hibernate: Fix pm_hibernation_mode_is_suspend() build breakage

Commit 495c8d35 ("PM: hibernate: Add pm_hibernation_mode_is_suspend()")
that introduced pm_hibernation_mode_is_suspend() did not define it in
the case when CONFIG_HIBERNATION is unset, but CONFIG_SUSPEND is set.

Subsequent commit 0a6e9e09 ("drm/amd: Fix hybrid sleep") made the
amdgpu driver use that function which led to kernel build breakage in
the case mentioned above [1].

Address this by using appropriate #ifdeffery around the definition of
pm_hibernation_mode_is_suspend().

Fixes: 0a6e9e09 ("drm/amd: Fix hybrid sleep")
Reported-by: KernelCI bot <bot@kernelci.org>
Closes: https://groups.io/g/kernelci-results/topic/regression_pm_testing/115439919

 [1]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>