drm/radeon/radeon_drv: Move prototypes to a shared headerfile

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

 62 | void radeon_driver_unload_kms(struct drm_device *dev)
 | ^~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_kms.c:105:5: warning: no previous prototype for ‘radeon_driver_load_kms’ [-Wmissing-prototypes]
 105 | int radeon_driver_load_kms(struct drm_device *dev, unsigned long flags)
 | ^~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_kms.c:619:6: warning: no previous prototype for ‘radeon_driver_lastclose_kms’ [-Wmissing-prototypes]
 619 | void radeon_driver_lastclose_kms(struct drm_device *dev)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_kms.c:634:5: warning: no previous prototype for ‘radeon_driver_open_kms’ [-Wmissing-prototypes]
 634 | int radeon_driver_open_kms(struct drm_device *dev, struct drm_file *file_priv)
 | ^~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_kms.c:705:6: warning: no previous prototype for ‘radeon_driver_postclose_kms’ [-Wmissing-prototypes]
 705 | void radeon_driver_postclose_kms(struct drm_device *dev,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~

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: Gareth Hughes <gareth@valinux.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>