video/sis: Remove unused structs SiS_SDRDRAM_TYPE/SiS_DDRDRAM_TYPE/SiS_DRAMType

This patch removes the unused structs SiS_SDRDRAM_TYPE and SiS_DDRDRAM_TYPE
from init.h

These are not used anywhere so we can delete them.

The SiS_DRAMType is identically defined in sis_main.c and only used
there so we can remove it here.

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