drm/vc4: txp: Turn the TXP into a CRTC of its own

The TXP so far has been leveraging the PixelValve infrastructure in the
driver, that was really two things: the interaction with DRM's CRTC
concept, the setup of the underlying pixelvalve and the setup of the shared
HVS, the pixelvalve part being irrelevant to the TXP since it accesses the
HVS directly.

Now that we have a clear separation between the three parts, we can
represent the TXP as a CRTC of its own, leveraging the common CRTC and HVS
code, but leaving aside the pixelvalve setup.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20f387f881b57f3474fa42d94cfd8bc1b7b80595.1591882579.git-series.maxime@cerno.tech