fbdev: intelfb: Fix potential divide by zero

[ Upstream commit d823685486a3446d061fed7c7d2f80af984f119a ]

Variable var->pixclock is controlled by user and can be assigned
to zero. Without proper check, divide by zero would occur in
intelfbhw_validate_mode and intelfbhw_mode_to_hw.

Error out if var->pixclock is zero.

Signed-off-by: Wei Chen <harperchen1110@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>