svga: Use __asm__ instead of asm

svga: Use __asm__ instead of asm

__asm__ is portable, and allows the svga driver to be compiled with the
c99 standard instead of requiring the gnu99 standard.

I have compile tested this with GCC and Clang on Linux.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Tested-by: Brian Paul <brianp@vmware.com>