st/dri: Fix driver loading if swrast isn't built

st/dri: Fix driver loading if swrast isn't built

If building hardware drivers only, then kms_swrast_create_screen
won't be defined in inline_drm_helper.h and hardware drivers will
fail to dlopen as a result.

Copy the #if guards from inline_drm_helper.h to dri_kms_init_screen
to make the definition/use of the function match.

Fixes radeonsi_dri.so dlopen with the following configure:

./configure --with-dri-drivers= --with-dri-driverdir=/usr/local/lib/dri/ \
--enable-gbm --enable-gallium-gbm --enable-debug --enable-opencl \
--enable-opencl-icd --with-gallium-drivers=radeonsi \
--with-egl-platforms=drm --enable-glx-tls --enable-texture-float \
--enable-omx

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>