Add option to disable XV hardware overlay

Add the "XVHWOverlay" boolean xorg.conf option to make it possible
to disable the XV acceleration feature using display layers on
sunxi hardware.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>
Update man page and README to reflect diverse platform support

Update the man page and bring it up-to-date, reflecting the fact
that the driver also supports non-sunxi platforms. Add description
of the "XVHWOverlay" option.

Also a small update to the README for similar reasons.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>
Don't initialize XV if we can't reserve a scalable sunxi-disp layer

In the case if an attempt to reserve a scalable sunxi-disp layer
failed, don't initialize XV at all. Otherwise any attempt to use
XV overlay is not going to work correctly and just results in
the following dmesg spam:

[  728.280000] [DISP] not supported yuv channel format:18 in img_sw_para_to_reg

This may happen on Allwinner A13 if scaler mode is enabled in
.fex file (A13 only has one DEFE scaler). Allwinner A10 also
can have similar troubles in dual-head configuration if scaler
mode is enabled for one or both screens (A10 has two DEFE scalers).

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>