staging: imx-drm: imx-tve: Check the return value of 'regulator_enable()'

Since commit c8801a8 (regulator: core: Mark all get and enable calls as
__must_check) we need to check the value returned by 'regulator_enable()'.

Do this check to get rid of the following build warning:

drivers/staging/imx-drm/imx-tve.c: In function 'imx_tve_probe':
drivers/staging/imx-drm/imx-tve.c:671:19: warning: ignoring return value of 'regulator_enable', declared with attribute warn_unused_result [-Wunused-result]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>