mach64: fix display corruption on big endian machines
The code for manual bit triple is not endian-clean. It builds the variable
"hostdword" using byte accesses, therefore we must read the variable with
"le32_to_cpu".
The patch also enables (hardware or software) bit triple only if the image
is monochrome (image->depth). If we want to blit full-color image, we
shouldn't use the triple code.