gfxstream: USE_MAGMA --> GFXSTREAM_ENABLE_HOST_MAGMA

Name change to be consistent with subsequent commit.

BUG=311486792
TEST=compile

Change-Id: If0f62c9d81ad16128f2b1a3c9b032e841d6fa28b
diff --git a/host/RenderThread.cpp b/host/RenderThread.cpp
index 74cee30..abe4fcc 100644
--- a/host/RenderThread.cpp
+++ b/host/RenderThread.cpp
@@ -303,7 +303,7 @@
         tInfo.m_vkInfo.emplace();
     }
 
-#if USE_MAGMA
+#if GFXSTREAM_ENABLE_HOST_MAGMA
     tInfo.m_magmaInfo.emplace(mContextId);
 #endif
 
@@ -566,7 +566,7 @@
             // try to process some of the command buffer using the Magma
             // decoder
             //
-#if USE_MAGMA
+#if GFXSTREAM_ENABLE_HOST_MAGMA
             if (tInfo.m_magmaInfo && tInfo.m_magmaInfo->mMagmaDec)
             {
                 last = tInfo.m_magmaInfo->mMagmaDec->decode(readBuf.buf(), readBuf.validData(),