drivers/video: fsl-diu-fb: remove broken reference count enabling the display

The functions enable_lcdc() and disable_lcdc() used the variable fb_enable
to keep a reference count of when the display is enabled, but the code is
broken.  The display is always disabled when disable_lcdc(), and it is
always enabled when enable_lcdc() is called, regardless of the value of
fb_enable.  Obviously, we don't need to keep a reference count, so just
remove the variable.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>