Legacy (non-atomic) page flip support is added to the driver by using the
mode_set_base CRTC function, that allows configuring a new framebuffer for
display. Since the function requires the primary plane's fb to be set
already, this is done prior to calling the function in the page flip helper
and reverted if the flip fails.

The vblank interrupt handler is also refactored to support passing an event.
The PIPE_TE_STATUS bit is also considered to indicate vblank on medfield
only, as explained in psb_enable_vblank.

It was tested by running weston on both poulsbo and cedartrail.
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106094400.445834-3-paul.kocialkowski@bootlin.com