drivers: gpu: Mark function as static in cdv_intel_dp.c

Mark function cdv_intel_fixed_panel_mode() as static in
drm/gma500/cdv_intel_dp.c because it is not used outside this file.

This eliminates the following warning in drm/gma500/cdv_intel_dp.c:
drivers/gpu/drm/gma500/cdv_intel_dp.c:680:6: warning: no previous prototype for ‘cdv_intel_fixed_panel_mode’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>