winsys/radeon: better explain the num_tile_pipes fixup for TAHITI (v2)

v2: Clarify the relation between num_tiles_pipes and GB_TILE_MODE and the fix
 needed for Tahiti as suggested by Marek.

Signed-off-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
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