drm/exynos: remove ignoring return value warning in hdmi

The definition of regulator_bulk_enable is fixed with __must_check
and this causes following build warning.
warning: ignoring return value of 'regulator_bulk_enable',
declared with attribute warn_unused_result
This patch fixes to check return value of the function.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>