virgl: add support for PIPE_TRANSFER_MAP_DIRECTLY

Treat it as mapping the guest storage directly.

This is necessitated by a recent change to st/mesa (commit
ce04fbf6) which assumes PIPE_TRANSFER_MAP_DIRECTLY is always
supported.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
13 jobs for !1400 with virgl-map-directly in 8 minutes and 23 seconds (queued for 4 seconds)
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passed debian #434538

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passed meson-arm64 #434547

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passed meson-armhf #434546

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passed meson-clover #434544

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passed meson-i386 #434548

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passed meson-swr-glvnd #434539

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passed scons-swr #434541

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passed scons-win64 #434542

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