MLK-17371 gpu: imx: dpu: framegen: Use better timeout value to wait for ENSTS

MLK-17371 gpu: imx: dpu: framegen: Use better timeout value to wait for ENSTS

The DPU spec tells us that we need to wait for all pending frames to
be completed when a display stream is disabled.  It turns out
that the hardcoded 60-microsecond timeout value is not enough for
some low refresh rate video modes, e.g., 1920x1080@24, which makes
the display stream be disabled incorrectly(leave the hardware an
incorrect machine status).  The SoC design indicates that there are
two pending frames to complete in the worst case.  This patch waits
for at most three frame duration(which is enough for sure) so that
the hardware may flush out all the pending frames.  In case the clock
subsystem provides us a pixel clock with wrong rate and causes the
timeout value be unreasonably long, we truncate it to wait for at
most three seconds.

Signed-off-by: Liu Ying <victor.liu@nxp.com>