OvmfPkg/PlatformBootManagerLib: Fix gcc-4.8 Ia32 build failure
OvmfPkg/PlatformBootManagerLib: Fix gcc-4.8 Ia32 build failure

The Width and Height assignment suppresses an invalid gcc-4.8 warning
on Ia32.
These warnings look unjustified to me. Namely, near the beginning of
the function, there is a while(1) loop. In that loop,
ConvertBmpToGopBlt() is called unconditionally. If the call fails,
the rest of the loop body is not reached (where the Height and Width
variables are used -- the compiler warns about their use in the
switch statement). If the call succeeds, then the variables are set.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <9fa3a6894a36bca79e6a72ad65af5a5702efecd1@intel.com>
Reviewed-by: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>
Tested-by: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>

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