SSD1289 SSD2119: fixed warning messages

Few type castings in order to suppress warnings (-Wsign-compare):
"comparison between signed and unsigned integer expressions"
"signed and unsigned type in conditional expression"

I use GCC ARM Embedded with GCC 4.7.3 and have those warnings with regular ChibiOS Makefile.

I did this change only in SSD1289 and SSD2119 because I know enough only those two.