drm/imx: fixes and cleanups

Remove unused functions and empty callbacks, let the dw_hdmi-imx driver
reuse imx_drm_encoder_parse_of() instead of reimplementing it, replace
the custom register spinlock with the regmap default spinlock and remove
redundant tracking of enabled state in imx-tve, drop the explicit
drm_mode_config_cleanup() call in imx-drm-core, reduce the scope of edid
length variables that are not otherwise used in imx-ldb and
parallel-display, fix a memory leak in the parallel-display bind error
path, and drop an extraneous type qualifier from of_get_tve_mode().
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