mesa: Remove C++11 narrowing warnings

mesa: Remove C++11 narrowing warnings

Add -Wno-narrowing to CXXFLAGS for gcc.

It is safe to add this flag even for versions of gcc that don't recognize
it.  From the GCC Manual [1]: "[GCC] allows the use of new -Wno- options
with old compilers".

This removes warnings of the form
    warning: narrowing conversion of X from 'int' to 'float' inside { } is
    ill-formed in C++11 [-Wnarrowing]
in ff_fragment_shader.cpp and gen6_blorp.cpp of the form.  When building
i965, I observed no other difference in the build output.

[1] http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>