drm/fb-helper: drop redundant pixclock check from drm_fb_helper_set_par()

The fb_check_var hook is supposed to validate all this stuff. Any
errors from fb_set_par are considered driver/hw issues and resulting
in dmesg warnings.

Luckily we do fix up the pixclock already, so this is all fine.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230404194038.472803-2-daniel.vetter@ffwll.ch