nvfx: Move src/gallium/drivers to beginning of SCons include path.

nvfx: Move src/gallium/drivers to beginning of SCons include path.

libdrm-2.4.20 and earlier include the nouveau/nouveau_class.h header. A
later version of libdrm will not ship this header. Mesa also has this
header at src/gallium/drivers.

The symbol NV34TCL_VTXFMT_TYPE_HALF is needed by nvfx_vbo.c. This symbol
is not in the libdrm copy of the header but is in the Mesa copy of the
header. This patch moves src/gallium/drivers to the beginning of the
include paths such that when building on hosts with libdrm-2.4.20 or
ealier the build uses the copy in Mesa.