drm/msm/hdmi: fix HDMI_MUX_EN gpio request typo

HDMI_MUX_EN gpio is requested. If an error occurs, the same name
should be printed (HDMI_MUX_EN) instead of HDMI_MUX_SEL (typo).

Signed-off-by: Beeresh Gopal <gbeeresh@codeaurora.org>
Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Rob Clark <robdclark@gmail.com>