cairo_dri: check for 1080p mode if primary mode is 2160p

For displays where the primary mode is 2160p, check to see
if a 1080p mode exists, and if so, use it. This allows laptops
with native 4K panels to specify a 1080p video mode via kernel
command line and have fbwhiptail use that vs native resolution.

Test: boot Librem 15v4 with kernel param 'video=eDP-1:1920x1080'
Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>

cairo_dri.c

@@ -188,6 +188,18 @@ cairo_dri_t *cairo_dri_open(const char *dri_filename)
       struct drm_mode_crtc *saved_crtc = &(device->screens[device->num_screens].saved_crtc);
       device->screens[device->num_screens].conn = conn.connector_id;
       device->screens[device->num_screens].mode = conn_modes[0];
+      /*
+       * HiDPI scaling for 4k laptop screens
+       * only enabled when 1080p video mode also set via kernel param
+       */
+      if (conn_modes[0].vdisplay == 2160) {
+        /* try to find a 1080p mode */
+        for (i = 0; i < conn.count_modes; i++)
+          if (conn_modes[i].vdisplay == 1080) {
+            device->screens[device->num_screens].mode = conn_modes[i];
+            break;
+          }
+      }
       device->screens[device->num_screens].crtc = crtc;
       saved_crtc->crtc_id = crtc;
       if (ioctl(device->dri_fd, DRM_IOCTL_MODE_GETCRTC, saved_crtc) == 0)