drm/radeon: Clean up reference counting and pinning of the cursor BOs

Take a GEM reference for and pin the new cursor BO, unpin and drop the
GEM reference for the old cursor BO in radeon_crtc_cursor_set2, and use
radeon_crtc->cursor_addr in radeon_set_cursor.

This fixes radeon_cursor_reset accidentally incrementing the cursor BO
pin count, and cleans up the code a little.

Cc: stable@vger.kernel.org
Reviewed-by: Grigori Goronzy <greg@chown.ath.cx>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>