Bus: send DMA-inaccessible sources (PSRAM / flash on ESP32) through the CPU path

[ainyan03]

pushImageDMA() hands the caller's pointer straight to the bus. On ESP32 the SPI DMA cannot read PSRAM or flash, so an image held in PSRAM is sent as garbage. Sprites are not affected: pushSprite() already checks heap_capable_dma() before asking for DMA.

The SPI bus and the ESP32-S2 / ESP32-S3 parallel buses now check heap_capable_dma() on the source and fall back to their CPU (copy) path when it is false.

Target	PSRAM source	Flash source
ESP32	CPU (was DMA → garbage)	CPU (was DMA)
ESP32-S3 and other GDMA targets	DMA (unchanged)	CPU (was handed to the DMA as external RAM)
ESP32-S2	CPU (was DMA without writing the data cache back)	CPU (was DMA)

Internal RAM sources keep using DMA on all targets. addDMAQueue() drains the queue and waits before the CPU output, as the existing flash-encryption fallback already does, so a queued descriptor never reads a reused flip buffer.

Reproduction (ESP32 with 2 MB quad PSRAM, 135x240 ST7789 on SPI, Arduino core 3.x with -DBOARD_HAS_PSRAM)

auto buf = (uint16_t*)heap_caps_malloc(w * h * 2, MALLOC_CAP_SPIRAM);
// fill buf ...
display.startWrite();
display.pushImageDMA(0, 0, w, h, buf);
display.endWrite();

Reading the full screen back with readRect() after a full-screen transfer:

	Internal RAM source	PSRAM source
Before	0 / 64800 pixels differ	64800 / 64800 differ
After	0 / 64800	0 / 64800 (CPU, 13.4 ms)

ESP32-S3 boards (octal and quad PSRAM) still read back identical and keep the PSRAM DMA timing.