drm/radeon/rv770: Move 'rv770_get_*()'s prototypes to shared header

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

 drivers/gpu/drm/radeon/rv770_dpm.c:47:18: warning: no previous prototype for ‘rv770_get_ps’ [-Wmissing-prototypes]
 47 | struct rv7xx_ps *rv770_get_ps(struct radeon_ps *rps)
 | ^~~~~~~~~~~~
 drivers/gpu/drm/radeon/rv770_dpm.c:54:26: warning: no previous prototype for ‘rv770_get_pi’ [-Wmissing-prototypes]
 54 | struct rv7xx_power_info *rv770_get_pi(struct radeon_device *rdev)
 | ^~~~~~~~~~~~

Cc: Evan Quan <evan.quan@amd.com>
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: 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>