drivers: display: ssd1306: Invert colors at runtime

This allows to invert the display at runtime using the function
`ssd1306_set_pixel_format`. The current format is stored in the data
struct.

Also adjusted `ssd1306_get_capabilities` to return both supported
formats and the current one.

From the ssd1306 doc: 'In normal display a RAM data of 1 indicates an
"ON" pixel while in inverse display a RAM data of 0 indicates an "ON"
pixel.' Thus `PIXEL_FORMAT_MONO01 == SSD1306_SET_NORMAL_DISPLAY`.

Signed-off-by: Greter Raffael <rgreter@baumer.com>