fb: do not ignore fb_set_par errors

At the moment about half of the framebuffer drivers can return an error
code in fb_set_par. Until now it would be silently ignored by fbmem.c
and fbcon.c. This patch fixes fbmem.c to return the error code and
restore var on error.

But it is not clear in which video mode the device is when fb_set_par
fails.  It would be good and reasonable if it were in the old state but
there is no guarantee that this is true for all existing drivers.
Additionally print a message if a failing fb_set_par is detected in
fbmem.c or fbcon.c.

Although most errors should be caught by the previous fb_check_var some
errors can't as they are dynamic (memory allocations, ...) and can only be
detected while performing the operations which is forbidden in
fb_check_var.

This patch shouldn't have a negative impact on normal operation as all
drivers return 0 on success.  The impact in case of error depends heavily
on the driver and caller but it's expected to be better than before.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>