glsl: assert on incoherent point mode layout-id-qualifier validation

glsl: assert on incoherent point mode layout-id-qualifier validation

The point mode value in an ast_type_qualifier can only be true if the
flag is already set since this layout-id-qualifier can only be or not
be present in a shader.

Hence, it is useless to check for its value if the flag is already
set. Just replaced with an assert.

V2: assert instead of checking for coherence and raising a compilation
    error. Suggested by Timothy.

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>