drm/radeon/btc_dpm: Move 'evergreen_get_pi's prototype to shared header

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

 drivers/gpu/drm/radeon/rv770_dpm.c:62:30: warning: no previous prototype for ‘evergreen_get_pi’ [-Wmissing-prototypes]
 62 | struct evergreen_power_info *evergreen_get_pi(struct radeon_device *rdev)
 | ^~~~~~~~~~~~~~~~

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>