video/fbdev/stifb: Implement the stifb_fillrect() function

[ Upstream commit 9c379c65 ]

The stifb driver (for Artist/HCRX graphics on PA-RISC) was missing
the fillrect function.
Tested on a 715/64 PA-RISC machine and in qemu.

Signed-off-by: Helge Deller <deller@gmx.de>
Stable-dep-of: 776d875f ("fbdev: stifb: Fall back to cfb_fillrect() on 32-bit HCRX cards")
Signed-off-by: Sasha Levin <sashal@kernel.org>