revert: "fix SIGFPE in hwcomposer caused by modulo by zero"

Change-Id: I85730e2efd017cb558c990978b1e39ef8a594e10
hwc: Fix div_by_0 crash in hwcomposer

Inside sw_vsync generation loop, there is a logic error
while reading the vsync_rate. The vsync_rate should be re-read
after user sets the vsync_rate by calling start_sw_vsync
explicitly. This is guaranteed by re-reading it
ONLY after the vsync_cond is signalled.

Change-Id: I2da97e6cb972cc812f789c7f0fd621503679177d