fbsplash: use virtual y size in mmap size calculations

fbsplash: use virtual y size in mmap size calculations

The virtual y can be larger - and we can be even writing there since
we are taking into account the y offset. Avoids possible crash.
But use it only if set, seems it is not set if virtual area is not
allocated (though, often fbcon allocates some scrollback area).

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>