Support Linux DRM backend

Implemented DRM-based framebuffer setup, including:
 - Opening the DRM device
 - Creating dumb buffers and mapping them to framebuffers
 - Setting mode and handling CRTC for connected display output

The legacy DRM ioctl interface is used as the current implementation
does not require atomic mode-setting or page flipping.

Atomic mode-setting prevents inconsistent states, supports rollback on
failures, and enables mode testing before committing changes.

Atomic page flipping synchronizes multiple planes within a VBLANK
interval, preventing tearing and improving display updates, especially
for power-efficient embedded controllers.

While the atomic API enhances KMS with property-based state management,
its complexity is unnecessary for now. The legacy interface remains
adequate, but future needs may justify adopting atomic mode-setting.

Close sysprog21#60

Author: Bennctu

.vscode/settings.json

@@ -0,0 +1,5 @@
+{
+    "files.associations": {
+        "fcntl.h": "c"
+    }
+}

Makefile

@@ -123,6 +123,15 @@ libtwin.a_files-y += backend/fbdev.c
 libtwin.a_files-y += backend/linux_input.c
 endif
 
+ifeq ($(CONFIG_BACKEND_DRM), y)
+BACKEND = drm
+TARGET_LIBS += -ludev
+libtwin.a_files-y += backend/drm.c
+libtwin.a_files-y += backend/linux_input.c
+libtwin.a_cflags-y += $(shell pkg-config --cflags libdrm)
+TARGET_LIBS += $(shell pkg-config --libs libdrm)
+endif
+
 ifeq ($(CONFIG_BACKEND_VNC), y)
 BACKEND = vnc
 libtwin.a_files-y += backend/vnc.c