fbdev/cg6: Use initializer macro for struct fb_ops

Initialize struct fb_ops to the correct default values with the
internal __FB_DEFAULT_SBUS_OPS_*() macros. Rename the ioctl and
mmap callbacks to use the infix _sbusfb_. This makes them fit the
SBUS helpers' naming pattern.

It is not possible to use set all defaults via FB_DEAFULT_SBUS_OPS(),
as cg6 provides its own implementation of fb_fillrect, fb_copyarea,
and fb_imageblit. Hence we also avoid FB_SBUS_HELPERS, but let the
driver select its dependencies individually instead.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230806120926.5368-8-tzimmermann@suse.de