video: fbdev: amiga: Simplify amifb_pan_display()

The fb_pan_display() function in the core already takes care of
validating most panning parameters before calling the driver's
.fb_pan_display() callback, and of updating the panning state
afterwards, so there is no need to repeat that in the driver.

Remove the duplicate code.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Helge Deller <deller@gmx.de>