fbdev: atafb: Restrict SuperBlitter to supported formats

The SuperBlitter operations derive an integer byte count per pixel.  The
accelerated fill path handles only one-, two- and four-byte pixels.
However, the operations are currently installed for every external
framebuffer in SuperVidel RAM, including planar 1/2/4/8-bpp and 24-bpp
truecolor modes accepted by the external video parser.

For 1/2/4-bpp modes, the byte count becomes zero, so accelerated copies do
nothing and fills fall through to 32-bit stores.  Planar 8-bpp uses an
incompatible memory layout.  For 24-bpp modes, fills also use 32-bit stores
despite advancing addresses by three bytes per pixel.  These cases can
corrupt the framebuffer beyond the requested rectangle.

Enable the SuperBlitter operations only for the layouts they implement:
8-bpp packed pixels and 16/32-bpp truecolor.  Keep the existing software
operations for all other external formats.

Fixes: d463633d63e6 ("fbdev: atafb: Add support for SuperVidel's SuperBlitter")
Signed-off-by: Linmao Li <lilinmao@kylinos.cn>
Tested-by: Miro Kropáček <miro.kropacek@gmail.com>
Reviewed-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>