clients/simple-dmabuf-feedback: Remove unsupported f suffixes

NVIDIA is more pedantic than Mesa and correctly complains that `1.0f`
is not valid syntax in the OpenGL ES Shading Language version 1.00.
And we are indeed using SL version 1.00 by virtue of using an ES 2.0
context.

So use the syntax compatible with the context we've created.

Signed-off-by: Daniel van Vugt <daniel.van.vugt@canonical.com>