drm/vc4: Match drm_dev_enter and exit calls in vc4_hvs_lut_load

Commit 52efe36 ("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: 52efe36 ("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>

drivers/gpu/drm/vc4/vc4_hvs.c

@@ -225,7 +225,7 @@ static void vc4_hvs_lut_load(struct vc4_hvs *hvs,
 		return;
 
 	if (hvs->vc4->gen == VC4_GEN_4)
-		return;
+		goto exit;
 
 	/* The LUT memory is laid out with each HVS channel in order,
 	 * each of which takes 256 writes for R, 256 for G, then 256
@@ -242,6 +242,7 @@ static void vc4_hvs_lut_load(struct vc4_hvs *hvs,
 	for (i = 0; i < crtc->gamma_size; i++)
 		HVS_WRITE(SCALER_GAMDATA, vc4_crtc->lut_b[i]);
 
+exit:
 	drm_dev_exit(idx);
 }