drm/vc4: Match drm_dev_enter and exit calls in vc4_hvs_atomic_flush

drm/vc4: Match drm_dev_enter and exit calls in vc4_hvs_atomic_flush

Commit 92c17d16476c ("drm/vc4: hvs: Ignore atomic_flush if we're disabled")
added a path which returned early without having called drm_dev_exit.

Ensure all paths call drm_dev_exit.

Fixes: 92c17d16476c ("drm/vc4: hvs: Ignore atomic_flush if we're disabled")
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241008-drm-vc4-fixes-v1-2-9d0396ca9f42@raspberrypi.com
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>