feat(gfx): added driver-owned displays and a flush ioctl to /dev/gfxfb

[FlareCoding]

Summary

• /dev/gfxfb could only expose the boot framebuffer, which the display hardware reads on its own. A display driver that keeps its pixels in RAM shows them only when told what to copy.
• Such a driver registers a display in place of the boot framebuffer, GFXFB_FLUSH hands it the rects userland changed, and a gfxfb_info flag tells userland when to flush. The boot framebuffer path is unchanged.

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Note

Medium Risk
New user/kernel ioctl path with uaccess and mmap cache behavior changes; limited to graphics device plumbing with validation and tests.

Overview
Extends /dev/gfxfb so a display driver can register a driver-owned RAM surface (via register_display) instead of relying only on the boot framebuffer that hardware scans directly.

Userland still maps and draws the same way, but GFXFB_GET_INFO now exposes a GFXFB_INFO_NEEDS_FLUSH flag when a driver display is active, and a new GFXFB_FLUSH ioctl passes clipped dirty rectangles from user memory to the driver’s display::flush() in bounded batches. On the boot path, flush is a no-op; mmap now picks write-combining vs normal cache from surface::cacheable.

Adds kernel unit tests covering info flags, rect clipping/batching, malformed flush args, mmap attributes, and single-registration semantics.

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