drm/amdgpu/gfx10: fix programming of SC_HIZ_TILE_FIFO_SIZE field

max fifo size is 128 and PA_SC_FIFO_SIZE[20:15]=SC_HIZ_TILE_FIFO_SIZE
field is programmed in units of two entries, but 6 bits is insufficient
to hold value 128/2 = 64, so set this field as 0 which is interpreted by
the hardware as maximum physical fifo size(128).

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jack Xiao <Jack.Xiao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>