drm/amd/display: Fix wrongly passed static prefix

Currently, gcc spews a warning as:
  drivers/gpu/drm/amd/amdgpu/../display/dc/dcn10/dcn10_hubbub.c: In function ‘hubbub1_verify_allow_pstate_change_high’:
  ./include/drm/drm_print.h:316:2: warning: ‘debug_data’ may be used uninitialized in this function [-Wmaybe-uninitialized]

This is because the code checks against a static value although it's
basically a constant and guaranteed to be set.

This patch changes the type prefix from static to const for addressing
the compile warning above and also for letting the compiler optimize
better.

Fixes: 62d591a8 ("drm/amd/display: create new files for hubbub functions")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>