i2c: designware-pci: Cleanup driver power management

The PCI part of the DesignWare I2C driver does a lot of things that are not
required anymore. For example drivers aren't supposed to handle PCI state
transitions themselves. This is all provided by the PCI bus core already.

In addition to that there is no point scheduling RPM suspend on driver's
idle hook but instead we can use RPM autosuspend for this (which is enabled
in the driver already).

As a bonus, this patch also fixes following compile warning which is
emitted when the driver was compiled without CONFIG_PM_RUNTIME set:

drivers/i2c/busses/i2c-designware-pcidrv.c:245:12: warning: ‘i2c_dw_pci_runtime_idle’ defined but not used [-Wunused-function]

Reported-by: xinhui.pan <xinhuix.pan@intel.com>
Reported-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>