msm: mdss: hdmi: Replace of_get_gpio usage with of_get_named_gpio

HDMI Tx driver parses platform gpio data based on index, using
of_get_gpio api. Replace this index based lookup with name based
lookup, using of_get_named_gpio api.

Change-Id: Ic65aae1583c8c6f852c31fd454ac651fc6c8b907
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>