drm/bochs: Add screen blanking support

On bochs DRM driver, the execution of "setterm --blank force" results
in a frozen screen instead of a blank screen.  It's due to the lack of
the screen blanking support in its code.

Actually, the QEMU bochs vga side can switch to the blanking mode when
the bit 0x20 is cleared on VGA_ATT_IW register (0x3c0), which updates
ar_index in QEMU side.  So, essentially, we'd just need to clear the
bit at pipe disable callback; that's what this patch does essentially.

However, a tricky part is that the access via VGA_ATT_IW is done in
"flip-flop"; the first write is for index and the second write is for
the data like palette.  Meanwhile, in the current bochs DRM driver,
the flip-flop wasn't considered, and it calls only the register update
once with the value 0x20.

The spec and the actual VGA implementation in QEMU suggests that the
flip flop flag is discarded by reading the CRTC index register
(VGA_IS1_RC, 0x3da).  So, in this patch, we add the helper to read a
byte and the call to clear the flip flop flag before changing the
blank / unblank setup via VGA_ATT_IW register.

v1->v2:
* discard ar_flip_flop by reading 0x3da, add bochs_vga_readb()
* include video/vga.h for VGA register definitions
* move the blank/unblank code to bochs_hw_blank()

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421080859.14869-1-tiwai@suse.de