viafb: cleanup viafb_cursor

Clean the hardware cursor handling up.

The most notable change is that it no longer buffers the values in
viacursor but uses the ones in cursor instead as they are guaranteed to be
always valid.

Furthermore it uses local instead global variables where possible, moves
the cursor variable in shared as only one hardware cursor is supported and
returns an error if memory allocation fails.  Last but not least it fixes
a too small buffer (as u32 has only 4 and not 32 bytes) but this did not
produce any known problems.

This is mostly a code cleanup, no negative runtime changes are expected.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Scott Fang <ScottFang@viatech.com.cn>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: Harald Welte <laforge@gnumonks.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>