drm/i915/selftests: Rename pm_ prefixed functions names

pm_resume and pm_suspend might be conflict with the ones defined in
include/linux/suspend.h. Rename all pm_* to igt_pm_* in selftests since
they are only used here.

v2 by Jani:
- Use igt_ prefix instead of i915_ to avoid colliding with existing
  i915_pm_* functions
- Rename all pm_ prefixed functions in the file

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210420130853.10573-1-jani.nikula@intel.com