video: fbdev: pmag-aa-fb: Adapt to current APIs

Rework the driver to use the current frambuffer and TURBOchannel APIs,
including proper resource management and using the new framework for
hardware cursor support.

NB two Bt431 cursor generators are included onboard, both responding at
the same TURBOchannel bus addresses and with their host data buses wired
to byte lanes #0 and #1 respectively of the 32-bit bus.  Therefore both
can be accessed simultaneously with 16-bit data transfers.  Cursor
outputs of the chip wired to lane #0 drive the respective overlay select
inputs of the Bt455 RAMDAC, whereas cursor outputs of the chip wired to
lane #1 drive the respective P3 pixel select inputs of the RAMDAC.

So 5 (out of 17) Bt455 color registers are usable with this board:
palette entries #0 and #1 for frame buffer pixel data driven while
neither cursor generator is active, palette entries #8 and #9 for frame
buffer pixel data driven while cursor generator #1 is active only and
the overlay entry while cursor generator #0 is active.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>