drm/amd/display: fix regamma build optimization

[Why]
When the global variable pow_buffer_ptr is set to -1, by definition
optimizations should not be used to build the regamma. Since
translate_from_linear_space unconditionally increments this global, it
inadvertently enables the optimization.

[How]
Increment pow_buffer_ptr only if it is not -1.

Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>