drm/i915/pm: drop intel_suspend_hw()

All intel_suspend_hw() does is clear PCH_LP_PARTITION_LEVEL_DISABLE bit
in SOUTH_DSPCLK_GATE_D for LPT LP. intel_suspend_hw() gets called from
i915_drm_suspend().

However, i915_drm_suspend_late() calls
intel_display_power_suspend_late(), which in turn calls hsw_enable_pc8()
on HSW and BDW. The first thing that does is clear
PCH_LP_PARTITION_LEVEL_DISABLE bit in SOUTH_DSPCLK_GATE_D.

Remove the duplicated clearing of the bit, effectively delaying it from
i915_drm_suspend() to i915_drm_suspend_late(), and remove the
unnecessary intel_suspend_hw() function altogether.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f732a7922c2450b41169c9b79a80fba97ab00592.1677678803.git.jani.nikula@intel.com