drm/radeon: add missing header dependencies

We get a few warnings when building kernel with W=1:
drivers/gpu/drm/radeon/radeon_clocks.c:35:10: warning: no previous prototype for 'radeon_legacy_get_engine_clock' [-Wmissing-prototypes]
drivers/gpu/drm/radeon/atombios_encoders.c:75:1: warning: no previous prototype for 'atombios_get_backlight_level' [-Wmissing-prototypes]
drivers/gpu/drm/radeon/r600_cs.c:2268:5: warning: no previous prototype for 'r600_cs_parse' [-Wmissing-prototypes]
drivers/gpu/drm/radeon/evergreen_cs.c:2671:5: warning: no previous prototype for 'evergreen_cs_parse' [-Wmissing-prototypes]
....

In fact, these functions are declared
in drivers/gpu/drm/radeon/radeon_asic.h,
so this patch adds missing header dependencies.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>