The mipi_dsi_dcs_nop() function returns an error but we weren't
checking it in boe_panel_prepare(). Add a check. This is highly
unlikely to matter in practice. If the NOP failed then likely later
MIPI commands would fail too.
Found by code inspection.
Fixes: 812562b8d881 ("drm/panel: boe-tv101wum-nl6: Fine tune the panel power sequence")
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>