st/mesa: fix warnings about implicit conversion on enumeration type

These enums match but compiler warns about implicit conversion.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
15 jobs for !511 with fix-some-st-warnings in 13 minutes and 30 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #191093
ubuntu

00:01:07

 
  Build+Test
passed #191094
autotools

00:12:22

passed #191096
meson-clang

00:07:57

passed #191102
meson-gallium-clover-llvm

00:06:29

passed #191103
meson-gallium-clover-llvm39

00:08:23

passed #191101
meson-gallium-drivers-other

00:08:02

passed #191100
meson-gallium-radeonsi

00:06:22

passed #191104
meson-gallium-st-other

00:06:19

passed #191095
meson-gallium-swr

00:12:02

passed #191099
meson-glvnd

00:06:12

passed #191098
meson-loader-classic-dri

00:07:16

passed #191097
meson-vulkan

00:04:51

passed #191106
scons-llvm

00:08:22

passed #191105
scons-nollvm

00:07:56

passed #191107
scons-swr

00:11:13