drm/simpledrm: Compute linestride with drm_format_info_min_pitch()

If not given, compute the stride with drm_format_info_min_pitch(). It's
the standard helper for this purpose.

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Fixes: fd9e3169 ("drm/simpledrm: Compute framebuffer stride if not set")
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220922130944.27138-2-tzimmermann@suse.de