fbdev: c2p: Use BUILD_BUG() instead of custom solution

Replace the call to the custom non-existing function by a standard
BUILD_BUG() invocation.

Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20200112171521.22443-1-geert@linux-m68k.org