drm/xe/display: Use acpi_target_system_state only if ACPI_SLEEP is enabled

drm/xe/display: Use acpi_target_system_state only if ACPI_SLEEP is enabled

This fixes the build error below with CONFIG_ACPI_SLEEP=n:

	drivers/gpu/drm/xe/xe_display.c:334:23: error: implicit declaration of function ‘acpi_target_system_state’; did you mean ‘acpi_get_system_info’? [-Werror=implicit-function-declaration]
	  334 |         bool s2idle = acpi_target_system_state() < ACPI_STATE_S3;

Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>