mskd: Don't use MFX_FOURCC_RGB565 if it's undefined

../subprojects/gst-plugins-bad/sys/msdk/msdk.c(61): error C2065: 'MFX_FOURCC_RGB565'

The minimum required version for the format seems to MFX_VERSION >= 1028
8 jobs for fix-msdk-build in 23 minutes and 27 seconds (queued for 53 seconds)
Status Job ID Name Coverage
  Preparation
passed #147080
gst indent

00:00:35

passed #147079
manifest

00:00:24

 
  Build
passed #147081
build fedora x86_64

00:05:21

 
  Test
passed #147082
check fedora

00:03:24

 
  Full Builds
passed #147083
build android arm64 api28

00:04:37

passed #147085
build cerbero cross win32

00:09:04

passed #147086
build cerbero cross win64

00:05:04

passed #147084
build cerbero fedora x86_64

00:03:58