Call drmmode_do_crtc_dpms from drmmode_crtc_dpms as well

Leo pointed out that drmmode_do_crtc_dpms wasn't getting called when
turning off an output with

 xrandr --output <output> --off

This meant that the vblank sequence number and timestamp wouldn't be
saved before turning off the CRTC in this case.

Reported-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
(Ported from amdgpu commit ceeacb455cd058492a493aac954deab8455804b5)
Acked-by: Alex Deucher <alexander.deucher@amd.com>