v3d: Add GL_ARB_vertex_array_bgra support

This is done by adding support to PIPE_FORMAT_B8G8R8A8_UNORM, and
relying on the R/B swapping for vertex attributes implemented in the
compiler.

v2:
 - Simplify the loop (Iago)

v3:
 - Assert before derreferencing variable (Iago).

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Closes: #3078
Part-of: <!7196>
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