backend/drm: fix black screens when enabling output
backend/drm: fix black screens when enabling output

This patch fixes this failure:

    01:57:16.642 [ERROR] [backend/drm/drm.c:360] Failed to page-flip output 'eDP-1': a page-flip is already pending
    01:57:16.684 [ERROR] [backend/drm/drm.c:360] Failed to page-flip output 'eDP-1': a page-flip is already pending
    01:57:16.684 [ERROR] [backend/drm/drm.c:732] Failed to initialize renderer on connector 'eDP-1': initial page-flip failed
    01:57:16.684 [ERROR] [backend/drm/drm.c:805] Failed to initialize renderer for plane
    01:57:16.684 [sway/config/output.c:423] Failed to commit output eDP-1

References: https://github.com/swaywm/sway/issues/5101

ascent12/wlroots,SirCmpwn/wlroots,ascent12/wlroots,swaywm/wlroots,swaywm/wlroots