freedreno/a4xx: lower srgb in shader for astc textures

freedreno/a4xx: lower srgb in shader for astc textures

This *seems* like a hw bug, and maybe only applies to certain a4xx
variants/revisions.  But setting the SRGB bit in sampler view state
(texconst0) causes invalid alpha for ASTC textures.  Work around this
by doing the srgb->linear conversion in the shader instead.

This fixes 392 dEQP tests: dEQP-GLES3.functional.texture.*astc*srgb*

(The remaining fails seem to be a bug w/ ASTC + linear filtering, also
possibly a420.0 specific.)

Signed-off-by: Rob Clark <robclark@freedesktop.org>