fbdev: sm712fb: use 1024x768 by default on non-MIPS, fix garbled display

Loongson MIPS netbooks use 1024x600 LCD panels, which is the original
target platform of this driver, but nearly all old x86 laptops have
1024x768. Lighting 768 panels using 600's timings would partially
garble the display. Since it's not possible to distinguish them reliably,
we change the default to 768, but keep 600 as-is on MIPS.

Further, earlier laptops, such as IBM Thinkpad 240X, has a 800x600 LCD
panel, this driver would probably garbled those display. As we don't
have one for testing, the original behavior of the driver is kept as-is,
but the problem has been documented is the comments.

Signed-off-by: Yifeng Li <tomli@tomli.me>
Tested-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Teddy Wang <teddy.wang@siliconmotion.com>
Cc: <stable@vger.kernel.org>  # v4.4+
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>