drivers: display: display_mcux_elcdif: Rework double framebuffer

Improved the eLCDIF driver implementation:
* Optional (working) double framebuffers
* Statically allocated framebuffer(s)
* Removed obsolete Kconfig entries

Also modified the mimxrt1170 EVK default config.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>