Fixed the review issues for pm_rc6_residency IGT case

Fixed the review issues for pm_rc6_residency IGT case

Why need add rc6_residency_counter subtest case:
RC6 feature support residency counter,from power consumption aspect,
the counter closer to 1,the better.If the counter is < 0.9, the residency
is not good and will impact power consumption value, if the counter is  > 1,
sysfs file is inaccurate.

Attach the test result message:
root@x-bdw05:/GFX/Test/Intel_gpu_tools/intel-gpu-tools/tests# ./pm_rc6_residency
IGT-Version: 1.6-g9a70e29 (x86_64) (Linux: 3.15.0-rc7_drm-intel-nightly_0a37b5_20140604+ x86_64)
Subtest rc6-residency-check: SUCCESS
This machine doesn't support rc6pp
This machine doesn't support rc6p
The residency counter : 0.987000
This machine entry rc6 state.
Subtest rc6-residency-counter: SUCCESS

root@x-bdw05:/GFX/Test/Intel_gpu_tools/intel-gpu-tools/tests# ./pm_rc6_residency --run-subtest rc6-residency-counter
IGT-Version: 1.6-g9a70e29 (x86_64) (Linux: 3.15.0-rc7_drm-intel-nightly_0a37b5_20140604+ x86_64)
This machine doesn't support rc6pp
This machine doesn't support rc6p
The residency counter : 0.987000
This machine entry rc6 state.
Subtest rc6-residency-counter: SUCCESS

root@x-bdw05:/GFX/Test/Intel_gpu_tools/intel-gpu-tools/tests# ./pm_rc6_residency --run-subtest rc6-residency-check
IGT-Version: 1.6-g9a70e29 (x86_64) (Linux: 3.15.0-rc7_drm-intel-nightly_0a37b5_20140604+ x86_64)
Subtest rc6-residency-check: SUCCESS

root@x-bdw05:/GFX/Test/Intel_gpu_tools/intel-gpu-tools/tests# ./pm_rc6_residency --list
rc6-residency-check
rc6-residency-counter

Run as non-root
[haha@x-pk home]$ ./pm_rc6_residency
IGT-Version: 1.6-g18d2130 (x86_64) (Linux: 3.13.0-rc3_drm-intel-nightly_639e4d_20131210+ x86_64)
No intel gpu found
Subtest rc6-residency-check: SKIP
Subtest rc6-residency-counter: SKIP

Run on non-intel platform
[root@x-pk5 home]# ./pm_rc6_residency
IGT-Version: 1.6-g18d2130 (x86_64) (Linux: 3.13.0-rc3_drm-intel-nightly_639e4d_20131210+ x86_64)
Test requirement not met in function read_rc6_residency, file pm_rc6_residency.c:77:
Last errno: 2, No such file or directory
Test requirement: (!(file))
Subtest rc6-residency-check: SKIP
Subtest rc6-residency-counter: SKIP

Signed-off-by: Wendy Wang <wendy.wang@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>