drm/amd/display: remove unused struct 'dc_reg_sequence'

'dc_reg_sequence' was added in
commit 44788bbc ("drm/amd/display: refactor reg_update")

but isn't actually used.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>