drm/fbdev-client: Skip DRM clients if modesetting is absent

Recent generations of Tegra have moved the display components outside of
host1x, leading to a device that has no CRTCs attached and hence doesn't
support any of the modesetting functionality. When this is detected, the
driver clears the DRIVER_MODESET and DRIVER_ATOMIC flags for the device.

Unfortunately, this causes the following errors during boot:

    [      15.418958] ERR KERN drm drm: [drm] *ERROR* Failed to register client: -95
    [      15.425311] WARNING KERN drm drm: [drm] Failed to set up DRM client; error -95

These originate from the fbdev client checking for the presence of the
DRIVER_MODESET flag and returning -EOPNOTSUPP. However, if a driver does
not support DRIVER_MODESET this is entirely expected and the error isn't
helpful.

Prevent this misleading error message by setting up the DRM clients only
if modesetting is enabled.

Changes in v2:
- use DRIVER_MODESET check to avoid registering any clients

Reported-by: Jonathan Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250613122838.2082334-1-thierry.reding@gmail.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>

drivers/gpu/drm/clients/drm_client_setup.c

@@ -4,6 +4,7 @@
 
 #include <drm/clients/drm_client_setup.h>
 #include <drm/drm_device.h>
+#include <drm/drm_drv.h>
 #include <drm/drm_fourcc.h>
 #include <drm/drm_print.h>
 
@@ -33,6 +34,10 @@ MODULE_PARM_DESC(active,
  */
 void drm_client_setup(struct drm_device *dev, const struct drm_format_info *format)
 {
+	if (!drm_core_check_feature(dev, DRIVER_MODESET)) {
+		drm_dbg(dev, "driver does not support mode-setting, skipping DRM clients\n");
+		return;
+	}
 
 #ifdef CONFIG_DRM_FBDEV_EMULATION
 	if (!strcmp(drm_client_default, "fbdev")) {