V4L/DVB (6691): pvrusb2: Rework pipeline state control

This is a new implementation for video pipeline control within the
pvrusb2 driver.  Actual start/stop of the pipeline is moved to the
driver's kernel thread.  Pipeline stages are controlled autonomously
based on surrounding pipeline or application control state.  Kernel
thread management is also cleaned up and moved into the internal
control structure of the driver, solving a set up / tear down race
along the way.  Better failure recovery is implemented with this new
control strategy.  Also with this change comes better control of the
cx23416 encoder, building on additional information learned about the
peculiarities of controlling this part (this information was the
original trigger for this rework).  With this change, overall encoder
stability should be considerably improved.  Yes, this is a large
change for this driver, but due to the nature of the feature being
worked on, the changes are fairly pervasive and would be difficult to
break into smaller pieces with any semblence of step-wise stability.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>