drm/i915: remove duplicate typedef for intel_wakeref_t

Fix the duplicate typedef for intel_wakeref_t leading to Clang build
issues. While at it, actually make the intel_runtime_pm.h header
self-contained, which was claimed in the commit being fixed.

Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Cc: Nathan Chancellor <natechancellor@gmail.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
References: http://mid.mail-archive.com/20190521183850.GA9157@archlinux-epyc
References: https://travis-ci.com/ClangBuiltLinux/continuous-integration/jobs/201754420#L2435
Fixes: 0d5adc5f

 ("drm/i915: extract intel_runtime_pm.h from intel_drv.h")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190522103505.2082-1-jani.nikula@intel.com