package/mesa3d: rename the prompt

Rename the prompt for mesa3d so that it matches what we usually do
for prompts of packages: just name the package in lower case.

This will help generate nicer tables in the generated packages list,
both in existing tables, and in the upcoming table of virtual packages.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>