drm/mcde: dsi: Fix invalid pointer dereference if panel cannot be found

The "panel" pointer is not reset to NULL if of_drm_find_panel()
returns an error. Therefore we later assume that a panel was found,
and try to dereference the error pointer, resulting in:

    mcde-dsi a0351000.dsi: failed to find panel try bridge (4294966779)
    Unable to handle kernel paging request at virtual address fffffe03
    PC is at drm_panel_bridge_add.part.0+0x10/0x5c
    LR is at mcde_dsi_bind+0x120/0x464
    ...

Reset "panel" to NULL to avoid this problem.
Also change the format string of the error to %ld to print
the negative errors correctly. The crash above then becomes:

    mcde-dsi a0351000.dsi: failed to find panel try bridge (-517)
    mcde-dsi a0351000.dsi: no panel or bridge
    ...

Fixes: 5fc537bf ("drm/mcde: Add new driver for ST-Ericsson MCDE")
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118130252.170324-1-stephan@gerhold.net