meson: libdrm shouldn't appear in Requires.private: if it wasn't found

meson: libdrm shouldn't appear in Requires.private: if it wasn't found

Otherwise, using pkg-config to retrieve flags will fail, e.g.

$ pkg-config gl --cflags
Package libdrm was not found in the pkg-config search path.
Perhaps you should add the directory containing `libdrm.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libdrm', required by 'gl', not found

Fixes: 3218056e0eb3 ("meson: Build i965 and dri stack")
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>