drm/radeon: Move prototypes to shared header

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/radeon_kms.c:756:5: warning: no previous prototype for ‘radeon_get_vblank_counter_kms’ [-Wmissing-prototypes]
 756 | u32 radeon_get_vblank_counter_kms(struct drm_crtc *crtc)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_kms.c:826:5: warning: no previous prototype for ‘radeon_enable_vblank_kms’ [-Wmissing-prototypes]
 826 | int radeon_enable_vblank_kms(struct drm_crtc *crtc)
 | ^~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_kms.c:853:6: warning: no previous prototype for ‘radeon_disable_vblank_kms’ [-Wmissing-prototypes]
 853 | void radeon_disable_vblank_kms(struct drm_crtc *crtc)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>