viafb: improve pitch handling

Split the pitch handling up and replaces the calculation from virtual xres
and bpp with fix.line_length which already contains the pitch and does not
add any constrains for the virtual resolution.

Also add a bit to the second pitch which the documentation mentions but
which was ignored by the driver.

Although it is a bit unclear what the right pitch for some LCD modes is
this patch should have no negative runtime impact.

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