drm/i915: remove HAS_RUNTIME_PM check from RPM get/put/assert helpers

We don't really need to check this flag in the get/put/assert helpers,
as on platforms without RPM support we won't ever enable RPM. That means
pm.suspend will be always false and the assert will be always true.

Do this to simplify the code and to let us extend the RPM asserts to all
platforms for a better coverage.

Motivated by Ville.

v2-v3:
- unchanged
v4:
- remove the HAS_RUNTIME_PM check from intel_runtime_pm_enable() too
  made possible by the previous two patches
v5:
- rebased on the previous new patch in the series that keeps
  HAS_RUNTIME_PM() in intel_runtime_pm_enable() with a permanent
  reference taken there

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v3)
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450352931-16498-1-git-send-email-imre.deak@intel.com