drm/stm: ltdc: Fix leak of px clk enable in some error paths

The pixel clock gets enabled early during init, since it's required
in order to read registers. This pixel clock must be disabled if
errors during this init phase.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500277223-29553-2-git-send-email-philippe.cornu@st.com