drm/solomon: Simplify mode_valid() using DRM helper

drm/solomon: Simplify mode_valid() using DRM helper

The ssd130x_crtc_mode_valid() function contains a manual implementation
to validate the display mode against the panel's single fixed resolution.

This pattern is common for simple displays, and the DRM core already
provides the drm_crtc_helper_mode_valid_fixed() helper for this exact
use case.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Iker Pedrosa <ikerpedrosam@gmail.com>
Link: https://lore.kernel.org/r/20250920-improve-ssd130x-v2-3-77721e87ae08@gmail.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>