mesa: Fix build empty DRI_DRIVER

In some cases, such as on sparc64, armv6 and armv7 etc, DRI_DRIVER,
GALLIUM_DRIVER and/or VULKAN_DRIVER might be undefined. This causes an
error similar to
make: "/usr/ports/graphics/mesa-dri/Makefile" line 106: Malformed conditional (${DRI_DRIVERS:M${:UI915}})

Fix this by checking if [DRI,GALLIUM,VULKAN]_DRIVER is defined before
checking what it is set to.

While here, remove textproc/py-mako dependency from mesa-dri/Makefile, it's
already included in mesa-dri/Makefile.common, no need to depend on it twice.

PR: 247117
Reported by: jbeich