cirrusfb: check_par fixes

1. Check if virtual resolution fits into memory.
   Otherwise, Linux hangs during panning.
2. When selected use all available memory to
    maximize yres_virtual to speed up panning
   (previously also xres_virtual was increased).
3. Simplify memory restriction calculations.

Signed-off-by: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>