media: mediatek: vcodec: mtk_vcodec_dec_hw: Use devm_pm_runtime_enable()

Convert pm_runtime_enable() to the managed version, and clean up error
handling and unnecessary .remove() callback accordingly.

Signed-off-by: Fei Shao <fshao@chromium.org>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Suggested-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>