Fix #13293: Flush rgb lcd PSRAM framebuffers after allocation (IDFGH-12244)

[seijikun]

Framebuffers are allocated using calloc, which also zero's the buffer. This zeroing is done by the CPU and thus uses the CPU's writeback cache for the PSRAM. If the framebuffer is not flushed after allocation, the DMA hardware is going to read stale data and push that to the display hardware.

This fixes: #13293

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[suda-morris]

Thanks for the fix!

[suda-morris]

sha=38f46583a46d6e44239ee1c16852d27b760d8e3d

[suda-morris]

merged in c755a62

we will backport this fix to 5.2/5.1/5.0 soon