drm/omapdrm: use BUG_ON macro for error debugging.

Since the if statement only checks for the value of the `id` variable,
it can be replaced by the more concise BUG_ON() macro for error
reporting.
Issue found using coccinelle.

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200102095515.7106-1-wambui.karugax@gmail.com