Changed format constants to match what vmwgfx driver expects

Changed format constants to match what vmwgfx driver expects

When allocating a new buffer using pipe driver, pipe constants
are used to define a format based on corresponding HAL_PIXEL_FORMAT
However, vmwgfx driver expects other constants
(SVGA3D_A8R8G8B8, SVGA3D_X8R8G8B8, SVGA3D_R5G6B5, etc)

Changed the returning constants to match what driver expects.

v2: [cwhuang]
 - Use the constants defined in svga3d_types.h.
 - Use C99 standard to avoid building errors with mesa 10.6.