Reading modes from EDID EEPROMs requires both I2C and EDID support to
be available. As the simple panel driver is also useful without mode
reading support, only built it in if the requirements are met.

Fixes:
In function `simple_panel_get_modes':
simple-panel.c:88: undefined reference to `of_find_i2c_adapter_by_node'
simple-panel.c:91: undefined reference to `edid_read_i2c'
simple-panel.c:95: undefined reference to `edid_to_display_timings'

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>