drm/i915: remove leftover i915_gem_pm.h declarations from i915_drv.h

Remove the duplicates.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/48f5ace6393533372da5d13df3de0203c8db11b3.1644507885.git.jani.nikula@intel.com