backend/drm: get physical size from libdrm
backend/drm: get physical size from libdrm

Values from libdrm are likely more reliable than raw values from the EDID. We
were already using values from libdrm, but they were overwritten by parse_edid.
See drm.c:

    wlr_conn->output.phys_width = drm_conn->mmWidth;
    wlr_conn->output.phys_height = drm_conn->mmHeight;

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