If there is no panel node in DT and instead display timings are provided
directly in FIMD node, there is no panel object created and ctx->panel
becomes NULL. However during Exynos DRM initialization
drm_helper_hpd_irq_event() is called, which in turns calls
exynos_dpi_detect(), which dereferences ctx->panel without a check,
causing a NULL pointer derefrence.

This patch fixes the issue by adding necessary NULL pointer check.
Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>