drm/arm: make undeclared items static

drm/arm: make undeclared items static

Make the following items static to avoid clashes with
other parts of the kernel (dev_attr_core_id) or just
silence the following sparse warning:

drivers/gpu/drm/arm/malidp_drv.c:371:24: warning: symbol 'malidp_fb_create' was not declared. Should it be static?
drivers/gpu/drm/arm/malidp_drv.c:494:6: warning: symbol 'malidp_error_stats_dump' was not declared. Should it be static?
drivers/gpu/drm/arm/malidp_drv.c:668:1: warning: symbol 'dev_attr_core_id' was not declared. Should it be static?

Signed-off-by: Ben Dooks (Codethink) <ben.dooks@codethink.co.uk>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191017111756.12861-1-ben.dooks@codethink.co.uk