fbdev: ssd1307fb: Drop optional dependency

Only a single out of three devices need a PWM, so from driver it's
optional. Moreover it's a single driver in the entire kernel that
currently selects PWM. Unfortunately this selection is a root cause
of the circular dependencies when we want to enable optional PWM
for some other drivers that select GPIOLIB.

Fixes: a2ed00da

 ("drivers/video: add support for the Solomon SSD1307 OLED Controller")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Helge Deller <deller@gmx.de>