[Intel-gfx] hsw rps values regress RPS on Macbook Air

[Eric Anholt]

On my new MBA with danvet's drm-intel-next-queued, I'm not getting
working RPS.  vblank_mode=0 glxgears never ups the frequency, and
vblank_mode=0 openarena only makes it up to 500mhz.  Reverting
1ee9ae3244c4789f3184c5123f3b2d7e405b3f4c gets the machine to responsive
RPS: fully on while the GPU is busy, fully lowered when it's not.

Since we're always just looking for all-on or all-off and never see
workloads that actually want to be somewhere in between, could we please
just move to race to idle for RPS?

dmi info:

Handle 0x001B, DMI type 1, 27 bytes
System Information
        Manufacturer: Apple Inc.
        Product Name: MacBookAir5,2
        Version: 1.0
        Serial Number: C02JH14TDRVG
        UUID: E58AA5BB-95BE-115D-AE6F-E12B41830529
        Wake-up Type: Power Switch
        SKU Number: System SKU#
        Family: MacBook Air
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