drm/i915: Fix an error handling in 'intel_framebuffer_init()'

We should go through the error handling path to decrease the
'framebuffer_references' as done everywhere else in this function.

Fixes: 2e2adb05

 ("drm/i915: Add render decompression support")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170910085642.13673-1-christophe.jaillet@wanadoo.fr