drm/i915: remove bogus mutex_unlock from error-path

This has been lost in the locking rework for intel_alloc_context_page:

commit 2c34b850
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Sat Mar 19 18:14:26 2011 -0700

    drm/i915: fix ilk rc6 teardown locking

Cc: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>