drm/armada: armada_output: Remove some unused functions

Removes some functions that are not used anywhere:
armada_drm_encoder_mode_fixup() armada_drm_encoder_commit() armada_drm_encoder_prepare()

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>