drm/simpledrm: Compute framebuffer stride if not set

Compute the framebuffer's scanline stride length if not given by
the simplefb data.

v3:
	* get pixel size from drm_format_info_bpp() (Geert, Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220801135028.30647-5-tzimmermann@suse.de