drm/amd/display: fix is_timing_changed() prototype

Three functions in the amdgpu display driver cause -Wmissing-prototype
warnings:

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_resource.c:1858:6: error: no previous prototype for 'is_timing_changed' [-Werror=missing-prototypes]

is_timing_changed() is actually meant to be a global symbol, but needs
a proper name and prototype.

Fixes: 17ce8a69 ("drm/amd/display: Add dsc pre-validation in atomic check")
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>