drm/i915: Remove intel_modeset_suspend_hw

Another layer of indirection for just an lpt-only w/a is a bit
excessive. Reduce it.

This was added in

commit 7d708ee4
Author: Imre Deak <imre.deak@intel.com>
Date:   Wed Apr 17 14:04:50 2013 +0300

    drm/i915: HSW: allow PCH clock gating for suspend

Cc: Imre Deak <imre.deak@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>