fbdev: imxfb: warn about invalid left/right margin

Warn about invalid var->left_margin or var->right_margin. Their values
are read from the device tree.

We store var->left_margin-3 and var->right_margin-1 in register
fields. These fields should be >= 0.

Fixes: 7e8549bc ("imxfb: Fix margin settings")
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Helge Deller <deller@gmx.de>