drm/pl111: Cleanup local header file

The header file contains prototypes for two nonexisting
functions. Get rid of them.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908124709.4758-1-linus.walleij@linaro.org