drm/amdgpu: fix checking pmops when PM_SLEEP is not enabled

'pm_suspend_target_state' is only available when CONFIG_PM_SLEEP
is set/enabled. OTOH, when both SUSPEND and HIBERNATION are not set,
PM_SLEEP is not set, so this variable cannot be used.

../drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c: In function ‘amdgpu_acpi_is_s0ix_active’:
../drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:1046:11: error: ‘pm_suspend_target_state’ undeclared (first use in this function); did you mean ‘__KSYM_pm_suspend_target_state’?
    return pm_suspend_target_state == PM_SUSPEND_TO_IDLE;
           ^~~~~~~~~~~~~~~~~~~~~~~
           __KSYM_pm_suspend_target_state

Also use shorter IS_ENABLED(CONFIG_foo) notation for checking the
2 config symbols.

Fixes: 91e27371 ("drm/amdgpu: Check pmops for desired suspend state")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-next@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org