Bug#977298: BUG: DPMS value not changing when requested with modetest & proptest (i915 driver)

[VDRU VDRU <user.vdr@gmail.com>]

Package: libdrm-tests
Version: 2.4.103-2

Using Debian Sid 64bit, kernel 5.9.6, i915 driver.. Tried using both
modetest and proptest to disable DPMS. Neither reported a problem but
the DPMS value remained unchanged.

$ modetest -c
trying to open device 'i915'...done
Connectors:
id      encoder status          name            size (mm)       modes   encoders
143     142     connected       HDMI-A-1        1600x900
 46      142
  modes:
        index name refresh (Hz) hdisp hss hse htot vdisp vss vse vtot)
  #0 3840x2160 60.00 3840 4016 4104 4400 2160 2168 2178 2250 594000
flags: phsync, pvsync; type: preferred, driver
  #1 4096x2160 60.00 4096 4184 4272 4400 2160 2168 2178 2250 594000
flags: phsync, pvsync; type: driver
.............trimmed for size
        2 DPMS:
                flags: enum
                enums: On=0 Standby=1 Suspend=2 Off=3
                value: 0
.............trimmed for size
$ modetest -w 143:DPMS:3
trying to open device 'i915'...done

Doing a `modetest -c` here gives identical results to the above.. Also:
$ cat /sys/class/drm/card0/card0-HDMI-A-1/dpms
On

Replacing modetest with proptest also gives identical results as above, with:
$ proptest 143 connector 2 3
trying to open device 'i915'...done