mmc: core: Fix card remove detect when use extcon

Since SDM845 uses extcon for card insertion and remove detection
and cd_gpio in device tree is not given, so the mmc_gpio_get_cd
function cannot detect card remove by reading gpio value. Fix it
by checking extcon state if extcon is initialized during sdhci
msm probe.

Change-Id: Ib44d211f707393cb440c387a06fa70d3adc99736
Signed-off-by: Can Guo <cang@codeaurora.org>