drm/radeon: Drop unnecessary #include <linux/vga_switcheroo.h>

This was added to three files even though they don't use any
vga_switcheroo symbols:

Added to radeon_acpi.c by commit d7a2952f ("drm/radeon: Add
support for the ATIF ACPI method to the radeon driver").

Added to radeon_asic.c by commit 0a10c851 ("drm/radeon: create
radeon_asic.c").

Added to radeon_bios.c by commit 6a9ee8af ("vga_switcheroo:
initial implementation (v15)").

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>