drm/i915: Remove i915_drm_suspend_mode

enum i915_drm_suspend_mode suspend_mode is only used in
intel_display_power, while we only care about whether we perform a
s2idle. Remove it and use a simple bool.

v2: Rebase

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com> # v1
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230529105900.1942814-1-jani.nikula@intel.com