drm/exynos: fix accidental on-stack copy of exynos_drm_plane

gcc rightfully complains about excessive stack usage in the fimd_win_set_pixfmt()
function:

drivers/gpu/drm/exynos/exynos_drm_fimd.c: In function 'fimd_win_set_pixfmt':
drivers/gpu/drm/exynos/exynos_drm_fimd.c:750:1: error: the frame size of 1032 bytes is larger than 1024 byte
drivers/gpu/drm/exynos/exynos5433_drm_decon.c: In function 'decon_win_set_pixfmt':
drivers/gpu/drm/exynos/exynos5433_drm_decon.c:381:1: error: the frame size of 1032 bytes is larger than 1024 bytes

There is really no reason to copy the large exynos_drm_plane
structure to the stack before using one of its members, so just
use a pointer instead.

Fixes: 6f8ee5c2

 ("drm/exynos: fimd: Make plane alpha configurable")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>