Fix sample_mask_in.bit_count_per_two_samples tests for 2x MSAA.

Fix sample_mask_in.bit_count_per_two_samples tests for 2x MSAA.

The dEQP-GLES31.functional.shaders.sample_variables.sample_mask_in.
bit_count_per_two_samples.multisample_{texture,renderbuffer}_{1,2}
tests report:

    Verifying gl_SampleMaskIn.
    Fragment shader may be invoked [ceil(numSamples/2), numSamples] times.
    => gl_SampleMaskIn should have the number of bits set in range
       [1, numSamples - ceil(numSamples/2) + 1]:

which suggests that maxBitCount should be ceil(2/2) + 1 = 1 + 1 = 2.

However, it then says:

    Setting minBitCount = 1, maxBitCount = 1.

Having two bits should be acceptable for 2x MSAA.

This patch drops the special case for 1x/2x MSAA, making them work
like all the other MSAA levels.