vc4: Avoid generating a custom shader per level in glGenerateMipmaps().

vc4: Avoid generating a custom shader per level in glGenerateMipmaps().

We were baking in the LOD of the source level to each shader.  Instead,
pass it in as a uniform -- this requires storing it to a temp register,
but that's better than compiling a ton of separate shaders:

total instructions in shared programs: 115032 -> 115036 (0.00%)
instructions in affected programs:     96 -> 100 (4.17%)
LOST:                                  572