drivers: display: ssd1306: Support connecting SPI and I2C at the same

Support connecting different display for each SPI and I2C
at the same time.

In a case like DTS below.

```
&spi1 {
        ssd1306_spi: ssd1306@0 {
		compatible = "solomon,ssd1306fb";
                ...
        };

};

&i2c0 {
        ssd1306_i2c: ssd1306@3c {
		compatible = "solomon,ssd1306fb";
		...
	};
};
```

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>