Massive interrupt storm when DPMS active

[Mike Pumford <mpumford%black-star.demon.co.uk@localhost>]

Running NetBSD-7.0-RC3 from a few days ago I've noticed that I get an 
interrupt storm but only when running X and DPMS has powered off the screen.

When the screen is blank the interupt rate on pin16 is around 36000 per 
seconds as shown on this systat vmstat snapshot:

36374 ioapic0 pin 16


When the screen is active:

    2 ioapic0 pin 16

This is an amd64 system:

cpu0 at mainbus0 apid 0: Intel(R) Core(TM) i7-4790K CPU @ 4.00GHz, id 
0x306c3

i915drmkms0 at pci0 dev 2 function 0: vendor 0x8086 product 0x0412 (rev. 
0x06)
drm: Memory usable by graphics device = 2048M
drm: Supports vblank timestamp caching Rev 2 (21.10.2013).
drm: Driver supports precise vblank timestamp query.
i915drmkms0: interrupting at ioapic0 pin 16 (i915)
intelfb0 at i915drmkms0
i915drmkms0: info: registered panic notifier
i915drmkms0: More than 8 outputs detected via ACPI
intelfb0: framebuffer at 0xffff80011c71a000, size 1920x1080, depth 32, 
stride 7680

XOrg reports the display as:

[    94.305] (--) intel(0): Integrated Graphics Chipset: Intel(R) HD 
Graphics 4600
[    94.305] (--) intel(0): CPU: x86-64, sse2, sse3, ssse3, sse4.1, 
sse4.2, avx, avx2
[    94.305] (II) intel(0): Creating default Display subsection in 
Screen section

I also seem to be getting a stream of these messages to the dmesg buffer:

DRM error in intel_uncore_check_errors: Unclaimed register before interrupt
DRM error in intel_uncore_check_errors: Unclaimed register before interrupt
DRM error in intel_uncore_check_errors: Unclaimed register before interrupt

These also stop when the DPMS is deactivated.

Any suggestions on how to fix this or any additional information I can 
provide to help fix this is appreciated.

Mike