video/sis: Annotate SiS_DRAMType as __devinitconst

SiS_DRAMType is const and only used by sisfb_post_300_rwtest which is
marked __devinit we can annotate SiS_DRAMType with __devinitconst and
move it into the file scope in order to not have it created on the
stack.
This patch decreases the compiled module size by about 100bytes.

And since hardcoded values are bad we use ARRAY_SIZE for determining
the size of SiS_DRAMType ;)

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>