ALSA: emu10k1: make sure synth DMA pages are allocated with DMA functions

Commit a5003fc0 ("[ALSA] emu10k1 - simplify page allocation for synth")
switched from using the DMA allocator for synth DMA pages to manually
calling alloc_page().
However, this usage has an implicit assumption that the DMA address space
for the emu10k1-family chip is the same as the CPU physical address space
which is not true for a system with a IOMMU.

Since this made the synth part of the driver non-functional on such systems
let's effectively revert that commit (while keeping the
__synth_free_pages() simplification).

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Takashi Iwai <tiwai@suse.de>