drm/amdgpu/dce8: 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 dce8 crtc_cursor_set2, and use
amdgpu_crtc->cursor_addr in dce8 set_cursor.

This fixes dce8 cursor_reset accidentally incrementing the cursor BO
pin count, and cleans up the code a little.

Port of radeon commit:
cd404af0

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>