fbdev: copyarea function taught to fully support swapped pixel order in byte

This correct case, when source and destination X coordinates difference is n
multiple of pixels in byte.  This is probably rare case, but this case should
supported for completeness.

Reorganization of FB_READL and FB_WRITEL calls results in code size decrease
for normal build without swapping support and size with support enabled is
reasonable too.

[adaplas]
Add missing fb_rev_pixels_in_long() prototype.

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>