drm/i915/display: Remove unused POWER_DOMAIN_MODESET

That power domain became unused after commit 41b4c7fe ("drm/i915:
Disable DC states for all commits").

Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230817175312.295559-1-gustavo.sousa@intel.com