i2c-i801: Simplify dependency towards GPIOLIB

Arbitrarily selecting GPIOLIB causes trouble on some architectures,
so don't do that. Instead, just make the optional multiplexing code
depend on CONFIG_I2C_MUX_GPIO instead of CONFIG_I2C_MUX for now. We
can revisit if the i2c-i801 driver ever supports other multiplexing
flavors.

Also make that optional code depend on DMI, as it won't do anything
without that.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Fengguang Wu <fengguang.wu@intel.com>