drm/vc4: Match drm_dev_enter and exit calls in vc4_hvs_lut_load

drm/vc4: Match drm_dev_enter and exit calls in vc4_hvs_lut_load

Commit 52efe364d196 ("drm/vc4: hvs: Don't write gamma luts on 2711")
added a return path to vc4_hvs_lut_load that had called
drm_dev_enter, but not drm_dev_exit.

Ensure we call drm_dev_exit.

Fixes: 52efe364d196 ("drm/vc4: hvs: Don't write gamma luts on 2711")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Closes: https://lore.kernel.org/dri-devel/37051126-3921-4afe-a936-5f828bff5752@samsung.com/
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-v1-1-9d0396ca9f42@raspberrypi.com
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>