drm/i915: Remove rpm asserts that use i915

Quite a few of the call points have already switched to the version
working directly on the runtime_pm structure, so let's switch over the
rest and kill the i915-based asserts.

v2: rebase

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613232156.34940-3-daniele.ceraolospurio@intel.com