viafb: support color depth 15 and 30

Add support for the color depth 15 on IGA1 and 30 on IGA1 and IGA2.  To
allow the usage of those the driver now refuses color depth that are
totally off and otherwise the selection in viafb_check_var is used.
Therefore the first call to this for the first framebuffer was delayed a
bit.  It only enables the new formats if they are requested exactly
(viafb_bpp=15|30).

As this is a new feature, no regressions are expected.  The color depth 15
was successfully tested.  Didn't get anything usable for 30 but that might
be the programs fault.  I would like to get some feedback whether it works
as expected or not if somebody knows a program/configuration where it
should.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: Scott Fang <ScottFang@viatech.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>