drm: mcde: Fix forgotten user of drm->dev_private

Unable to handle kernel NULL pointer dereference at virtual address 00000918
pgd = (ptrval)
[00000918] *pgd=00000000
Internal error: Oops: 5 [#1] SMP ARM
Modules linked in:
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.7.0-15001-gfa384b50b96b-dirty #514
Hardware name: ST-Ericsson Ux5x0 platform (Device Tree Support)
PC is at mcde_display_enable+0x78/0x7c0
LR is at mcde_display_enable+0x78/0x7c0

Fix this by using to_mcde() as in other functions.

Fixes: fd7ee85c ("drm/mcde: Don't use drm_device->dev_private")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200613223027.4189309-2-linus.walleij@linaro.org