fbdev: stifb: Provide valid pixelclock and add fb_check_var() checks

stable inclusion
from stable-v5.10.176
commit 38742635eda98fb5d3304d921cddaaa62d656814
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I87BGI

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=38742635eda98fb5d3304d921cddaaa62d656814



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commit 203873a5 upstream.

Find a valid modeline depending on the machine graphic card
configuration and add the fb_check_var() function to validate
Xorg provided graphics settings.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>